Wouldn't using an encrypted onion routed network put a bit of a pinch on the amount of DDOS traffic one could push? Not to mention that there is probably an upper bound to the amount of traffic any given exit-node is willing to allow for. How many exit nodes restrict outbound connections to common ports?
Anon's preferred tool are the array of open HTTP proxies available. They provide relatively weak anonymity, but in scale it works well enough. LEAs have to subpoena the proxy owner and their ISP for logs, usually the owner has very poor logging anyway, often they're based in a foreign jurisdiction... There just aren't the resources available to go after them in any real numbers. They can (and do) go after a couple of people to make an example of them, but they always seem to pick on the low-hanging fruit who didn't go through a proxy.
Quite the opposite. The journalist idea of "Anonymous" is very different from the insider idea of Anonymous. In the simplest terms, Anonymous is anyone, really. It's not limited to 4chan users (there are other chans, and the users sometimes overlap). 4chan itself is just a convenient place to recruit people to perform an attack and later use as a scapegoat.
As an example, the anti-Scientology protests were reported under the catch-all banner of Anonymous, but the people organizing it and hitting the streets were also from rival forums such as Something Awful. Funny enough, the protest movement is completely different from (and at odds with) the original catalyst: other Anons DDoSing Scientology websites for using the DMCA to take down a video on YouTube. As someone mentioned earlier in the comments, one core motivation is trolling, or "lulz," so under this so-large-its-useless umbrella term "Anonymous," there are groups sabotaging one another just because it's funny.
The core people organizing* these DDoS attacks come and go, but they are a dedicated few who have the time and skills to design such programs to distribute to the masses. As in the Time magazine hack, it was respectably sophisticated (this article does it justice: http://musicmachinery.com/2009/04/15/inside-the-precision-ha...). And yes, this core group does have botnets, as well as excellent social engineering skills to motivate *channers to download and execute their software.
I love watching people and organizations of all varieties underestimate just how tenaciously annoying they can be.
It's different, and that's the point I wanted to make. A botnet owner could use tor to hide his own computer's address, and let the bots flood freely. Even if a few were traced, he doesn't care. Tor gives him privacy (for his master computer) without compromising his attack potential.
A 4chan poster can't do the same thing. He's attacking from his own computer. Either he attacks through tor (slow) or he doesn't (unsafe). Hence why I thought the post above does not make sense for anon's case: he can't hide his traces without channelling the attack through Tor itself.