Why forum websites does not allow the signature links at first post?

[uthra]

Some forum websites allowing the signature link after posting 50 post or 30 post only.Why forum websites does not allow the signature links at first post?

[RH-Calvin]

This is just because to avoid spam. People sign up and starts spamming around just to increase their backlinks. This creates a negative on the Forum. So many forums allow signature links only after some number of quality posts.