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.
Unfollow - I think there can be reasons. For one thing, a big follower list becomes difficult to manage. Suppose you auto-follow, and decide to sample a couple of messages from every followee before you decide to keep following or unfollow. That is already a pretty high cost to pay, considering that it is not uncommon to get dozens of new followers every day (most of them scammers).
On the other hand, since I gave up on managing followers, I found that a lot of them sort themselves out by themselves: by the time I check if I want to follow them back, they have already been deleted from Twitter as spammers. Auto-unfollow could help in the same way, by removing those scammers who managed to evade deletion by Twitter.
Granted, I think Twitter frowns on auto-follow, too, but there are some valid reasons for it imo. Also in some cases it jut seems polite to follow back - think strange acquaintance from last party, if you don't have the guts to say "I don't really like you".
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.