Isn't this the second time this has happened though? I heard something about megaupload being shut down for a while back in November.
My point being that it might not be as permanent as it seems, last time didn't really stick either.
Ultimately, there's no legal backing to this action as the people who made megaupload haven't committed any crimes, if I hang up a sign for other people to post messages on and someone tapes stolen merchandise to it doesn't mean I committed a crime.
Recently the U.S. seems to ignore the law more and more and acting like its all justified, ranging from the war in Iraq, the assassination of an unknown individual later claimed to be Bin Laden (though most proof was conveniently destroyed and what remained was in the possession of the offending party) and now going to muscle their way into the current situation, which would be like administering the death penalty based on a claim before actually convicting him (remember, there was no lawsuit or anything to prove their claim to be true before they shut down the site)
So far, they have been getting away with breaking thousands of laws, some even constitutional (right to a trial is in your constitution as well as I recall) and I think it's well past time this nonsense is put to a stop before you completely turn to a police state.