yeah, I mean, this administration really doesn't seem to mind taking on significant costs - to the country I mean. I don't think the administration likes the idea of bearing significant costs themselves.
> 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.
If of course it's just a bargaining tool, you would think there is a quit pro quo.
Whilst the impact of a refusal to repatriate would be high remember the gold is probably not performing very much of a role, beyond a defensive one, and is nothing like the whole of their stocks. And the cost to US standing in enacting a refusal to NATO allies would be significant: You would expect future gold deposits to seek safer harbours for one, and the retention of the dollar as a hedge currency would be called into even more question.
In any case, it's a matter for law. On what grounds could the US refuse?