"Does the U.S. government even have the right to go and seize this homeless person's foreign assets?"
I suppose it would depend on the treaties with have with where those assets are held.
Also freeze =/= seize. They are similar and rhyme but not identical.
"Everyone seems to be conveniently forgetting that Megaupload has NOTHING to do with the United Stated except for operating on the Internet and not discriminating against U.S. customers."
Are you telling me that I can sit in Nowhereistan, hack into bank accounts all over the world to steal that money, transfer it into my Nowhereistan accounts and I'm immune to all the world's governments even though they have a treaty with Nowhereistan?