Please! You can NOT justify the difference
I don't need to. However as a forum owner I also grant my users the same right as I have become accustomed to. It's a matter of having some respect for the people that make a site what it is. It's also about being reasonable instead of being a nazi.
While on this subject and as Collective has clearly indicated, there's a big difference between posting a URL on it's own and plastering a thread with HEY COME HERE AND DOWNLOAD MAH L337 MP3 PLAYA <-- Used by a poster in another thread elsewhere here.
Signatures on posts are just a "This is me, this is where I come from". It's a measure put in by a vast majority of forum coders for this specific purpose. However the person that put something similar to what I said in allcaps in the other thread I mentioned thinks that this is a submission service. It's not. It's a discussion forum and nothing else.