Because people would steal them the way they steal towels and other things in hotel rooms? It'd be a crackhead's paradise -- find the nearest hotel, break two room windows in, and score a cool $600 worth of electronics in a few minutes. Score!
I think the hotels that would use this first would not be the motel in the drug district, but rather a high-rise in some city. If you can break into a 10th story window, I guess you can have the Roomba.
Currently, theft of this variety doesn't appear to be a real problem; there is tons of expensive stuff in hotel rooms right now. Video game systems, flat-screen TVs, mattresses, etc. The Roomba would be easier to steal, but I doubt it would become a common occurrence.
Besides, in my imagined version of how this would work, the door to the room would be open while the Roomba is doing its thing, and the maid would probably hear the window breaking. (There wouldn't be a Roomba in each room, the maid would just carry some when doing their cleaning, set them loose in a few rooms, wait for them to finish, repeat.)
Anyway, this just won't happen. I should have just written that initially and saved the bits used to represent this reply ;)