When you have an ultimatum, you're drawing a line in the sand saying either you're on this side or you're not. There's an inherent problem with this line of thinking.
Conflicts and situations are rarely black and white. By saying this you're forcing the decision making process to fit a this or that box. And rarely will that end will.
Not to mention this: When you're a kid and someone says DON'T TOUCH, what do you do? YOU TOUCH IT!!!!