You have to think of the goals of the desired user versus somebody trying to game the system. Twitter apparently wants users to not just think about building follower count.

Why would a casual user ever want to unfollow somebody just because that person has stopped following? The only possible reason is to use that now vacant follow on somebody who's more likely to follow them back, which is the exact strategy of scammers taken to an extreme level.

Incidentally, if Twitter really didn't want people trying to game the system, they probably shouldn't have made the system so much like a game.