Everything else -- the FBI fuckups, the legitimate users losing data, and so on -- is a secondary justification for supporting a warez site. Those issues can be opposed on their own without making the warezer-in-chief into a hero.
I could not have cared less about the man until I read about how little due process had been involved leading up to the raid on his home. There is something fundamentally wrong when one of the most powerful countries in the world can't use due process to arrest someone violating a copyright law.
He is (not by choice, but still) fighting the US government, particularly in an area where many perceive the latter to be bound by lobbies. That's enough to make him seem like a knight in shiny armor.
Personally, I'd like for both to lose ;)
Of course there is pirated stuff on MegaUpload, there's pirated stuff on every cloud hosting service.. every single one of them. Anecdotally, I use Google Docs and Dropbox to share music with people, should the government raid them and take all of their servers?
Whether or not you feel this is a piracy hub is beside the point, because if there was a reason to shut the site down, the people responsible should be able to present evidence of criminal activity in order to justify their actions.
The fact that they undertook a military raid, and tried to collect evidence after the fact is a clear sign that they did not have a strong case to begin with.
More than anything he's becoming a hero because the RIAA and MPAA have been using lawsuits to demand hundreds of thousands of dollars from middle-class parents, the elderly, and many others who are often innocent of crimes they are accused of. This has become a rallying point for many to stand up to these organizations tactics, because it's only been a one-sided battle using questionable practices for the past decade.