The reason for that, I assume, is to prevent spammers and phreaks from posting url's as soon as they register. It only takes a post or two for the mod's to catch and ban them. If they were allowed to register and post a url right away then they could use this forum and unsuspecting viewers to spread a virus, a trojan, spyware, ect...
The 10 post thing is to protect us from these spammers and phreaks of nature....