I'm not sure why Twitch gets such hate for this. His range even got nerfed because of it. He *isn't* good at it, merely "ok". Yi is FAR better at backdooring, as he can pop his ultimate and wuju and take a tower down without dieing *while inside it's range*.
I will agree that the ability some champs have to do this is counter-productive to the strategy of the game. While I don't think there's anything wrong with the concept (damage a tower while their team is elsewhere, without the aid of minions), being able to do more than half a tower's health in under 15 seconds is NOT something that should be allowed.
My basic suggestion is that towers have an ability:
All damage in excess of 1% of the tower's max hp is reduced to 1%. Then make Alistar's ability trigger after that (so he's the lone champ that can kill a tower in under 100 swings).
Sure you can still backdoor, but at 2.5 attack speed, it'll now take you 40 seconds worth of attacking. Or in other words, you'll have to backdoor the same turret more than once to kill it. Minion damage won't be affected at all, and it'll make it a *bit* slower for the 5 man push to take the tower down while a lone champ harasses them (and might allow the lone champ to get a kill if they don't deal with him).