I wouldn't expect the US to care too much about things like "NATO allies" with a head of state that expressed repeated desire to leave NATO.

> On what grounds could the US refuse?

"You need our trade more than we need your trade" is a very useful property to have when it comes to international law. Putting people you don't like on sanction lists, like people investigating your ally's war crimes, also seems to work pretty well. When push comes to shove, there's always "we have more guns than you", though American diplomatic action generally prefers to overthrow governments and install new, more US-aligned leadership as a head of state.

If it's a matter of law, you need someone to enforce that law.