It's customary to ask for a ridiculous clause like this up front, because no candidate will walk away over the mere sight of it. They get one free chance to screw you over, just like when you were talking about salary and they asked what you made at your last job. But you have to do your part; politely bounce it back.
You are a professional and this is a negotiation. No one will begrudge you putting a reasonable counteroffer on the table.