I think it was fairly obvious from the beginning why there is a rule about needing to buy something before using the restroom.
Many places you can just walk in and use the bathroom if it isn't a really busy restaurant or business or whatever. But for the businesses that are in a downtown area or a big city setting they need to make sure only customers are using their facilities or it will turn into a bunch of non-customers hanging out.
Most businesses aren't there for you to go over and hang out. Who'd thunk it eh