Actually lots of boards have that limitation and it used to be for a good reason.
There are two reasons why boards often require members reach a certain number of posts before they can upload their own -
1. When you upload your own avatar you are taking up hard disk resources. Many member come only to post for a few days and weeks and then leave. If everyone were to upload an avatar file onto the server it would take up a lot of disk space. So we limit that by making sure that only those who stick around long enough to reach 300 posts can do that. The rest can use avatars that are already on the disk, thus not taking up new disk space.
2. We wish to make sure that members know what the boards are all about and prevent situations where people may post inappropriate images in their avatars. As a matter of fact one thing we have considered is to set up a rule that only members with more than a certain number of posts will be able to post any images at all - to prevent trolls from posting inappropriate images.
Those I think were the reasons why Vbulletin comes with this option built in. Also, I believe most members enjoy "climbing" up the ladder of posts, acquiring new titles and with them new previleges. It's harmless fun and it does reflect how long you've been active with our community. Personally, I don't like "spam" posts just to increase one's rank and I definitely don't want to "trap" anyone into posting. I sure hope avatars, signatures and titles are not the main reasons why people join our community - their like side perks just for fun - no more