If you look around further there are also members who do not use a signature that are also spamming the forum..
1. Forum traffic through the usage of allowing users to promote websites encourages unhealthy discussion that leads to spam. I'm not so sure that that kind of traffic is what the forum really needs. If you have a restaurant and fill it with children, that's also traffic. But is it good traffic?
2. Members who do not use a signature and spam are most likely account farmers as well. There shouldn't be a reason for spam in the first place, anyway.