Matiqua, they are disabled for a couple of reasons.
Firstly, since the site became very prominent in Google searches we get a lot of visitors to the site as well as new members. The page views and posts have increased, which in turn has substantially increased bandwidth charges each month. Many signatures used graphics and animated icons, and some of them were consuming a large amount of bandwidth. DiscussionPool doesn't incur overage charges each month, so can afford to allow signatures.
Secondly, despite the Links & Images Posting Rules stating; "Posting a website URL regularly, or using it as a signature in posts, is regarded as spam, and the link will be removed.", some members continually used links to their own (or favourite) sites as signatures. Some of those sites contravened our rules and were putting our ICRA and Surfsafely listings at risk because of their content. This was also causing unnecessary work for the moderators.