I'm no defender of Mt. Gox but they are in Japan for a reason. Renting servers from a US based company could bring problematic legal issues. Although, coinlab seems to have found a way around it. Unfortunately the last I saw, they don't plan to come online with Mt. Gox until 2014.
I've only casually picked up some (possibly mis-)understanding over the years, but:
1) I believe that much of the infrastructure that Mt. Gox uses runs under Tiabans which Krapeles had going before he bought Mt. Gox, and much of it is co-located in the US. Including the forum upon which I am tapping out this note just now.
2) For all intents and purposes, Japan is a vassal of the US. If we say jump (or 're-start those fucking nuclear reactors'), the Japanese's only acceptable response is 'how high?'
3) Mt. Gox operates completely at the pleasure of Proxais...at least when the going gets tough. And they, like many other critical infrastructure companies, will dance to the tune that the US government calls.
4) Sufficiently motivated (usually be graft) the US has no trouble bringing huge swaths of the Internet to a full stop. As Kim Dotcom about how that works.
Sorry for the multiple SIC in names. I am to lazy to look things up right now.
