Generic top-level domain. Generic means generic, i.e. not country-related/controlled. If the US does not honour this principle, the rest of the world could create an alternate DNS system, which would be a disaster both for the US and for the rest of the world.
By the way, gov, edu and mil are not generic, they are US-only. In contrast, com, net and org are perfectly generic. For evidence, see the .us domain.