They do get kickbacks. It works something like this: A hotel books a certain number of rooms, say 50, and they give 2 comp rooms to the rooming department.
The comp rooms are first used for visiting bethel speakers and traveling overseers. But, there are usually so many comp rooms to go around, that they end up getting used by the Rank & File that are in the know.
In order to get a comp room as a Rank & File, you have to agree to a "suggested" donation for the hotel rate.
So, if you get the comp room, you are "suggested" to donate, say, $80/night (if that was the hotel's rate).