His website was accessible in the US by design (one could forbid US visitors in the ToS, and by geoip), and the "harm" from his supposed crime occurred, for those US users, in the US, not inside his server.
What sucks about this case is that it seems incredibly unlikely that linking constitutes a felony copyright infringement, because no copyrighted material is ever directly encountered. It seems much more likely that only a misdemeanor infringement occurred (if any), for which the UK does not extradite to the US.