Despite them fighting against groups like the MPAA in the past, it seems operating in the U.S. gives them too much liability and would rather stick to greener pastures:
Torrentspy's decision to stop accepting US visitors was NOT compelled by any Court rather it arises out of an uncertain legal climate in the United States regarding user privacy and the apparent tension between US and European Union Internet privacy laws.
You can read the announcement itself on the TorrentSpy blog, which is very light on other details.
This decision has probably been affected by a decision earlier this year in which a U.S. Judge demanded that TorrentSpy hand over data that could personally identify people searching for torrents. It was rumored then that a U.S. blockade to the site might occur.