It's because on autohosts you are allowed to call !map and change the map. This is because well... so you can change the map and not be playing on the same map over and over.
But you are not allowed to change the room type, because the teams room should stay teams
The result is that you can change the zerowars autohost to a map other than zerowars but you can't change the room type of the ULU autohost to something that you can have bots on
EDIT: as for why this hasn't been changed... probably because it's very complicated to do so and not high-prioity