There are lots of treaties and conventions on nuclear weapons and the laws of war that nations adhere to. If you made non-tactical nuclear weapons illegal I'm sure there would be some type of compliance although knowing whether compliance is genuine is debatable. It's no different than SALT and SALT II that limits nuclear warheads. We don't know if one or either side is cheating but both sides came to the table.
EDIT: Getting rid of non-tactical nuclear weapons would also mean making inter-continental ballistic missiles illegal since their only raison d'etre is to deliver such heavy and devastating payloads.
You missed my point, it does not matter how many treaties or laws there are that control these weapons. If there is no entity capable and willing to punish us for violating those treaties and laws, then those treaties and laws do not matter.
For example it does not matter if Bush comitted war crimes because no one is willing to try to charge him.