"Are people really stupid enough to use all their internet bandwidth to attack Sony because Microsoft told them to do so for removing OtherOS that nobody cared about?"
Microsoft did what now?
I won't get into that argument now, but some people did want the ability to both use OtherOs and play games on their PS3s, and were, quite rightly in my opinion, pissed off with Sony when they took it away.
And no, poeple weren't stupid enough to do this, because it isn't what happened. Sony got cracked, not DDoSed IIRC.
"Are people stupid enough to think the internet is anonymous?"
Yes, some are. I think naïve is probably a better word though. The people who assume that they are anonymous on the internet by default are probably the same people who believe without question the things they read in the papers or hear on Sky News. There are plenty of these people, unfortunately.
"Are people stupid enough to download malicious stuff like this and risk going to jail and/or getting really big fines?"
There have already been arrests, so I think you may have answered your own question.
I think the sheer fact that LOIC has been used for DDoS attacks, without proxy anonymisation, and that quite a lot of people have joined in to do so, demonstrates that there are number of people who are both stupid enough and either pissed off enough, or are trying to fit in with a 'cool' crowd, to do so.