Yes we do have a reason. A period of time not long ago spammers would sign up, put up a sign and spam the forums with links, pr0n among other various things.
It was decided back than that the signature is a privledge. Which it is. It doesnt have to be offered. Most site do it without thinking of how bad it can be abused.
We felt the force of how bad it can be. So we took steps to make sure that it doesnt happen. To stop it we made sure that people were going to stay and not drive by spam with them.
I hope that helps clear up the confusion.