I work in the hotel industry. Not only do large groups often receive complimentary rooms but some tour bookers receive a per room rebate. I recently had a tour operator request we quote prices including a $4 per room, per night rebate. I would not be at all surprised if the WT people do the same thing. The Society being a very large booker could probably demand a much higher rebate if they wanted to. I suspect the primary reason for requiring Witnesses to stay at Society approved hotels is because they want to get the "rebate" as well as strengthening their bargaining position by being the sole source of business for the Witnesses. "Its all about the benjamins." Plus they have the added benefit of keeping an eye on everyone if they all stay in the same hotel.
In a similar vein, remember the Seattle parking scandal where the Society was contractually prohibited from charging for parking so they changed it to a "donation" arrangement. That went over great too!