Basically because there's absolutely no-one who can stop them. The US largely doesn't abide by international law, refuses to join any international treaties that could result in their leaders being held to account in international courts, and basically just does whatever the feck it wants, because it can. They occasionally accept adverse rulings from mediation on economic issues (when Obama gave Iran their money back after about 50 years of sitting on it) it was purely because a court was about to rule they had to, but given that the US is also the primary funder of most of the international bodies its rare for them to have negative results.
This is one of the stupidest parts of Trump's isolationist stance and pulling funding from international organizations, the US basically OWN most of those organizations anyway. It's like having a crooked judge in your pocket, and then angrily denouncing him and saying you won't have any more cases put in front of him.