Originally, there were no restrictions on who could have a signature. This meant that anyone joining the forums, even people with malicious intent, could have a signature. After a while, it was changed to require 50 posts to get a signature. However, this caused many people to make many useless posts to get a signature. They decided on making people wait 15 days, as the theory was spammers wouldn't want to wait, and users wouldn't spam. However, the spammers did wait, and then there were 15 day old posts with spam in the signatures. These posts rarely got viewed by Scratchers, but did get indexed by search engines. Finally, the system was changed so that only users with Scratcher status can have a signature, the theory being that spammers wouldn't get the status. This is the way it still is.