It happened the same day everyone was protesting SOPA, right? So maybe this is the FBI protesting SOPA too, they're saying they don't need this new law, they have plenty of existing laws thank you very much, let's have a dawn raid.
SOPA and PIPA only allow the government to take down websites without going through a long legal process. It took a long legal process to shut down Megaupload, which did some questionable things, but the screwed up thing is that there's no doubt it was funded by UMG, who had a scuffle with Megaupload around a month ago.