I have often hear the argument from the hotels that adults typically use more of things in a room. A room with 4 adults in it will go through more soap, shampoo, coffee, towels etc than the same room with 2 adults and two kids in it. Is it $15 worth.....more than likely not but it is pretty standard practice no matter what resort or hotel you stay at. Also statistically a room with 4 adults in it also stands a greater chance of getting damaged and or being stripped clean of such things as towles, stationary, tolitries, light blulbs, furniture etc.
As far as hotels go though, Disney is very light on the extra fees. It is not uncommon to have such extras as parking, hospitality fee, resort fee, as well as mandatory bellman, safe and internet fees even if you do not use said services tacked onto your bill. And god help you if you take something from the min-bar or dare to pick up the phone and make a call.