It's cause Anonymous threatened to hack PSN if they didn't drop support (and we don't want another hack like last time)
Anonymous probably can't even hack the PSN. They weren't the ones who did the major hack that brought down the PSN for a month. Hell that group is still unnamed, they basically just did it for personal gain (aka for stealing credit cards and harvesting personal info) instead of the personal glory that Anonymous wants. If anything you'll just have the same petty "protest" that Sony got last time from Anonymous: they try to DDoS their site but fail, send Jack Tretton pizzas, and try to organize a sit-in that also fails. HOW FORMIDABLE.
This is probably just, as everyone said, a PR move. It seems that most gamers aren't in favor of the bill so protest or resistance might come from public support of the bill. Of course Sony and Nintendo support it since it would tackle any piracy on their systems, but doing it publicly could lead to some backlash from consumers. Last thing they want is to lose more sales to protest than from piracy.
EDIT: And don't say "OH PIRACY DOESN'T MAKE ANYONE LOSE SALES" because that's a load of bullshit. I know plenty of games I would've bought but didn't because I could just pirate them. I haven't bought DS games for like years and the last Wii game I bought was Goldeneye because it was on sale for $10, before that it was NMH2. I haven't even bought a PSP game. Like ever. On the other hand I still buy Xbox 360 games and even PS2 games when I can since I don't pirate on those systems. While it's not theft it certainly voids sales. I'm sure if you couldn't pirate on a console you'd end up buying more games for it.