If Megavideo and Megaupload were charging for the files they were allowing to be downloaded from their site and there wasn't a deal in place with the record companies for compensation of the rights holders and artists, it was piracy and they were infringing on copyrights of a LOT of different artists and song writers. That's what got Limewire shut down as well, I believe.
Yes, we have the right to protest peacefully according to the First Amendment but what they did is a federal crime. These people that are mild mannered employees by day and super hackers by night have big problems when they're discovered.
I don't disagree with that they did from a protest prospective. We have that right. But from an ethical standpoint, I can't condone it. Hacking is a crime. The intent behind hacking is always a malicious one- no matter how pure you claim your intentions are. Hacking into a foreign bank account in order to donate the millions of dollars to charity is an example- though it has nothing to do with the topic. It's just an example to make a point.
As long as the sites are insuring that the artists are getting paid somehow, I don't see them getting shut down- as long as the artists are getting compensated. Like everyone else in the world, actors, musicians, writers ect, expect to get paid. They have to make a living and survive too.