Signatures are different from the content of posts because:
a) Only established members are permitted to have them.
b) No one needs to read them. Any member can block Signatures.
c) If that's not enough you can block all posts from specific members.
There are also rules about congas in S.P.A.M. Why should some members be permitted to have their referral links in their signatures, generating multiple referral bonuses, when there's a rule about congas in one forum?