I actually think it's just posting crap under the sig deals that gets people banned. Probably half of newbs post crap and hollow, generic replies that don't contribute much but they'll probably be fine. Obviously if they become disruptive or go on a spamming/trolling spree that will probably change, but I think they're usually safe otherwise.
Well it's because of the fact posting equates to money that people have a propensity to spam. It's far easier for people to write short pointless posts rather than actually read a thread and try to post something that adds value. People have come to associate paid signatures with spamming as a result of this and hence it has a stigma attached.
Only saying "you are banned from posting" 
It is just not fare how scammers sometime are still on the forum and keep posting and i am banned and no one said why..