It's not about sovereignty, it's about the rule of law. Every international agreement involves governments committing themselves to do certain things and not do others. Sometimes that also means agreeing to abide by judgements by an arbitrator. But they're free to negotiate changes to those agreements or withdraw from them. That means they retain sovereignty.

Treaties passed by the legislative branch of the government are laws. Do you believe that your governments executive branch should be bound by the laws of your nation, or not?