They do operate legal entity in the US. At the very least, there were all those funds the Feds seized last year.

The U.S. doesn't necessarily have to enforce its laws overseas to go after Mt. Gox. Japan has its own consumer protection laws, and presumably they have something to say about how Mt. Gox has been operating, too. So filing suit in a Japanese court is also a perfectly legitimate option.

The US can enforce it laws on anybody it can get its hands on. For example Joaquin Guzman, the drug kingpin recently captured in Mexico faces charges in a variety of US states, even if he never set foot there. [1]

Mexico has to agree to hand him over, but once the US has him, they can happily convict him of crimes committed while he wasn't physically present in the US.

[1] http://www.npr.org/2014/02/25/282359473/arrest-of-el-chapo-n...

>The U.S. may enforce its laws upon U.S. entities, but it has no authority to do so on e.g. a Japanese one, unless a treaty/agreement exists that facilitates it.

The US has a history of extending its jurisdiction to practically anywhere if the Internet is involved. effective enforcement is another matter entirely, though.

>This is why China-only companies that engage in IP theft (or vice versa) aren't being hauled into court on the daily.

"IP theft" is pretty much impossible to achieve. You're thinking of infringement.