If it were only about WTS trying to help you save money, then they wouldn't care if you used expedia.com or velocity.com or hotels.com, if that gave a better deal. It seems evident (to borrow a WTS weasel phrase) that there must me some other motive.
Like any good investigation, a good place to start is by answering the question: "Who benefits?"
According to both an "in the know" post here at JWD several years ago, and also according to basic facts known to anyone that has worked in convention arrangement in the real world, the key issue is FREE ROOMS.
When you reserve a block of rooms for an event (any event: your wedding, etc), part of the negotiation is how many (and also, how luxurious) FREE ROOMS will be provided to the organizer, and what percentage of "fill" for the reserved block is required to get those FREE ROOMS.
So: it is in the interest of the WTS to fill the reserved blocks, to insure that the WTS Bethel senior staffers, GB members, etc. get their FREE ROOMS. That is why they insist everyone only call the hotels on the list, and identify that they want the JW block of rooms. If the blocks aren't filled to some pre-agreed-upon percentage level, then WTS does not get the FREE ROOMS.
This also explains why they continue to insist on this, year after year. If you take rooms "outside the arrangement" at either a hotel on the list, or one that is not on the list), the hotel(s) will figure it out, and next year decide they don't have to negotiate and offer the FREE ROOMS incentive. So the hotel doesn't negotiate the following year, and the FREE ROOMS disappear.
And we can't have WTS actually pay money for the hotel rooms for their senior staffers -- people that have taken a vow of poverty, and so can't afford it themselves. No, the WTS likes to keep the money flowing in one direction only: towards itself.
The hotel arrangement is about FREE ROOMS, not about saving money for the Rank & File.
~Q