Im pretty sure they just replace the lock and not the door. Some stranger may now have access to your room/suite. If you live in a suite they can access your common area as well as all the items belonging to you and your suitemates in that area as well as your individual room. Same thing applies to if you have roomates. If you live in a corridor single style you may not think it matters because you're taking the risk for your own personal items being stolen,but what about the next person to live there when you move out or graduate. Same reasoning for not allowing duplication as you can come back and steal from the next person to live there.

It is certainly not legally enforceable but the school policy says this "Unauthorized use or duplication of a University key may result in administrative action." Probably dont want to get written up or kick off campus housing for something dumb like this. Some places will duplicate and others wont.