This is a very common misconception. The US (and for that matter any other nation) is not 'under' the UN in any way. Sovereign states are just that. Sovereign.

The entire UN is basically a big committee where member governments get together and agree on 'stuff'. How things should be. How things are. etc. Thing is, not everyone agrees on every element of every treaty. Many states are not signatories to certain treaties (while they may be to others) basically saying 'That rule is bunk and we aren't following it'.

Of course, lots of other states push agreements into place or sign them and then have no intention whatsoever of actually following them. Saudi Arabia is a signatory to the treaty on the rights and status of women, for instance.

The UN, by its own charter, has no enforcement arm of any kind, nor is it ever allowed one. The only thing that can happen is a bunch of states get together and do whatever needs doing on their own possibly with the UN's blessing.

This is why 'International Law' is a joke. People think of it like criminal law, a set of rules put out by the government with an enforcement body to make sure they're followed. Really it' has no similarity to this at all.

It's far closer to international treaty law which for all intents and purposes says: "If you're strong enough to break this, the weaker party is humped unless they can get a big enough gang of friends together to do something about you."

It's more schoolyard bully than it is Cop arresting Murderer.