This is actually really interesting! I wonder how highly valued the post must be before the link is converted to dofollow. I think this is an okay way to prevent shameless link spam, but unless the limit for getting a dofollow is pretty high, it can be achieved easily by paying a few of the upvote bots.

Edit: I did some quick tests, and it currently seems that the dofollow kicks in at around the $9-$10 mark, at least on my posts.