The "WHO" and the "WHY" are closely related. The following reasoning will not solve either. It will however point out the many problems we got ourselves in with the Communication/Knowledge/Info revolution. The society needs to find its equilibrium and you will have a hard time finding a prophet who can see the end of  this process.

 1. Hackers: why? Because it is computer related and computer literacy is limited and will with the speed of development of technology always remain limited for the general public; for any individual to keep up with the tech. development, you need to be daily "hands-on" involved. It requires a lot of time to make a living, study all new developments and have "a life" too. This requires a special, strange, person: the hacker. The hacker has his own, special, priorities. Because his life is for some reason limited in some ways, for him time came free to do what a hacker needs to do to be a good hacker: study and hack the keyboard.
The roomer is that the used programs for the attacks are readily available on the internet. If I can find the time to try, just to see how easy or difficult it is to get such a program, I will do so and report to you.
It must be clear that a reasonable level of computer literacy is needed to find as well as to use such a program. And there needs to be a strong motivation by the perpetrator: 5 year prison for first time, 10 yr. for repeat law breaks and high fines. This leads to the second category on my list:

 2. Political Malcontents. Why?
The fall of the Soviet Union did not change the mind of many of the followers of the Communist belief system. And the effects on the American society are everywhere.
The dialectics of communism have permeated the language: many use the word "capitalism" where they should use "free enterprise system". This communist residue resents the blooming free enterprise economy. They are among us! They are motivated; they might have the knowledge.
They might or might not be involved. The experience is that political motivations can be very strong. And strongly bent: i.e. crooked. The goal justifies the means type of crooked! This kind of political motivation brings the individual down to the level of criminal. This leads me to the third category on my list, because the political malcontent becomes one with it.

 3. The Underworld. Why?
In the most free society on the Earth, the pursuit of happiness is conditional of the control of the individuals conscience. Because individual conscience is relative to the individuals relation with "absolute conscience" and uncontrolled self interest, that is selfishness, would bully the meek to a pulp, there has to be control through law. When the selfishness and the ignorance of the need for social behavior and responsibility, are not in balance, disrespect for law and society result in lawlessness. Therefor criminality always contains a mixture of the political (self interest) and the ignorance ingredient. Both factors contain again the aspect of lack of  relation to "absolute conscience". It exist in many forms, on many levels and gradations. 
So what has this to do with anything? It is possible that conditions experienced by, certain un kosher, like the sex site operators, causes one or more to make trouble. The whole sex buss was anyway always underworld. At this time they might not be motivated to attack their new medium. I guess it comes more from the political motivated direction. And political almost always means: Promoting Self Interest of a group or an individual, unless it is to promote freedom. The big Freedom; not the selfish MeMeMe freedom, but the real responsible Freedom that includes to be free of evil. 
Next Small Entrepreneurs: Why?

 4. Small Entrepreneurs. Why?
You might remember, some three years ago we heard, the internet was "the great equalizer"; "even a small company could look big on the internet". Well, now we know better: the big ones really are BIG.
They can and are spending big, they are employing big, they have quality content, they look beautiful graphically, they have great features, software with wonderful functionality, they have lots of adds and they rake in, again big. And in order to stay big and become even bigger, they spam us any way they can: On TV, on Radio, in the mail, in magazines, newspapers. Now the small entrepreneurs are told it is immoral, prohibited, not allowed etc., to spam. Only Email to persons and business you have already a relation with, can be sent. The internet is already so much part of the American economy, that when every small or tiny operation would be allowed to sent Email to whoever they want, the big ones would or could start to lose money, because of the traffic jam, on the information Highway, which infrastructure development is lacking behind the increase in number of users. And the big ones should not be hampered in any way, because they causing the prosperity of all of us, right? At least if you are in the stock market in some way, like a 401K plan, IRA's, etc.. 4% unemployment, is also good for everyone. However, wasn't the information highway meant for everyone too? Yes, but some are more equal than others. That's why I assume it is possible that some small operator really does not like to be hampered in what he wants to do, like Email to whoever he wants. If it turns out to have been that way, I would not be surprised. That, with the right understanding, should also fall in the category of  "Political" action, that is: promoting the interest of a particular group or individual. They are automatic also the next group:
(In the mean time some news is that University computers are from outside directed to do the dirty work of the perpetrators of this computer crime. My accusations of the influence of the anti business, anti free enterprise philosophy following professors, being the possible cause of misguided student action, had already come from my keyboard.)
5. The Competitors: why? 
Because the big ones are getting to big and claiming ownership of the E-road. They are so important to the American economy that the Att. Gen. Janet R. gives a press conference to introduce a special task force of the FBI, to take care of the matter. If you block the public road as an act of civil protest and disobedience, you get arrested and go in a cell for some hours and get a fine, 50 bucks or so. If you block "this" highway, you go to the slammer for a maximum of five years and fines of $100,000.- and up.
They have to find this kind of anonymous road blocker first, and that might not be that easy. "Let's keep things in proportion". There are people who are shorter in jail for murder; than again there are some on death row for same. Don't misunderstand me: I do not condone this kind of sabotage acts! I am projecting some view on the many aspects of this topic. At some time I have been shouted down for my opinion. I was to polite, to civil, to timid. Not anymore. I have opinions, about many things. I don't know who is doing this dirty deed! I am speculating on who qualifies for this roll of villain. Who else ? There are more candidates for the roll of villain. Let see what else we have:

6. Other Interest: why?
It is said that it is so easy to do, with "of the shelf software" I heard some nut expert told on CNNfn, that a 15 year old school kid could do this. But why? I don't see kids of 15 year with the motivation to disrupt E-commerce and other economical important operations. It might be an 18 year old college student, who is indoctrinated by one of those 60 th minded, late lefty professor, where the colleges and universities are crowded with. Well we can throw that kid in the slammer! We would not be able to do much to these kind of professors, who teach the students many other "harmful to society" ideas. That is a topic for next time. No, let me talk about in now. It is to important to pass it by for this less important topic. So next is a 
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