It's an anti-spam measure. Your first few posts are flagged if they have links, but as long as they aren't in violation of the site rules, they're approved when a mod or admin is online.
"What, exactly, is the internet? Basically it is a global network exchanging digitized data in such a way that any computer, anywhere, that is equipped with a device called a 'modem', can make a noise like a duck choking on a kazoo." - Dave Barry