You can modify your posts but you do need a certain amount of posts first. I believe it may be 100 posts but don't quote me, please.
You've got it.
The reason is that I found link spammers would create an account, make posts, then let the account sit unused for weeks or months. They would then go in and edit their posts to add the spam links - thus falling under the radar. The posts were not new, so they didn't rise to the top and get seen by human eyes, but the spam links were found by search engines.