How would they know - because most people leave them in their room - creating separation labor at the time of laundering (the mousekeepers don't have seperate bins for different kinds of laundry, so it all has to be retouched and sorted in the laundry room. That isn't difficult if you are pulling out sheets from towels, its a big deal if you are pulling one type of towel out from the other types). There isn't any reason for a cash guest to take pool towels to sub for bath towels - they get new towels every day if they want them.
Disney wants to keep hotel rates low, they'll use any excuse they can to pass expense onto DVC instead of impacting the profit on the hotel side.
Early on, if I remember the stories the old-timers told, OKW got daily housekeeping and towel service, like the hotels. They dropped it because its really expensive. DVC dues are high for most timeshares - understandable when you consider that transportation costs are wrapped in, along with other things non DVC timeshares don't need to provide. High dues makes it hard for Disney to sell DVC and it makes it hard for people to afford DVC. So they started charging for daily housekeeping and towels. If there wasn't a cost, they'd just change your towels ever day.