Also, affiliate and referral links are used on the signatures you see all over the place and no one will ever penalise you for creating your own signature and embedding the referral/affiliate links into it as well. I believe this still serves the same purpose as the person who embedded it in his posts directly instead of the signature design.
So in your opinion it's alright to wear a signature that leads to a site and then getting paid a fix amount, but it's not alright to have a special link for the same site to then earn as people click on it and use the service? Which one is better for you as a promoter, a fixed payment, or a dynamic one? Here is a better question, what if you are advertising your own site?
If you don't know the reason for restriction on ref links, here is why:
Scammers advertise for fake profit by promising good return, but in reality they just scam the unsuspected newbies. So it's not allowed to do that. Everything is in place for a reason, to avoid abuse.
Just what I was saying, I still see no valid reason for the restriction from what you pointed out here. Do you think people can't still embed their links in their posts with or without the forum allowing it? Of course, they do it. How does this stop the evil plan of those guys who are daredevils and only care about achieving their aim? Moderations and users reporting such are enough to settle it without affecting anyone. What you just said doesn't contain a good reason to me. Should we cut the head because of a headache?
Anything the forum says is fine by me since I have no external links to sell apart from my signature campaign, but at times, a rethink is a good forward too. Be it because of scams or not, people with bad intentions are still letting others know what they are selling in many sections of the forum. And if what you claimed is the true reason why it is not allowed, then the reason is not so nice. By doing this, you outrightly restrict those who are just posting legitimate company business while those who are posting their scams which you think you blocked still find their way to propagate their news. Moderation is key here.