The issue isnt if they have the legal right, we all know they do.
The real reason this is an issue is because the USA, and I think others, offered to help with weaponry on the condition that the weaponry provided would not be used to advance into Russia.
Now I think this condition was an unfair one, so I dont really mind that they ignored it. But the issue isnt about law its about what the USA should do, and what consequences there will be for the Ukraine. If it means we cut off support that may be the end for Ukraine.