I believe you require a few more posts for sending pm's to be available. So that new users signing up with 1 or 2 posts don't send you a million spam private messages. Not sure on the exact number but its not allot.
The reason for the rule is so that people must show some substance before they can PM people. In the past people have set up a username and have immediately started to harass people via PM. This is why the rule is in place.