I hadn't heard that the written permission sites weren't paid, could be true. I know all the other versions in my area are paid. As others said, hunt by reservation doesn't provide nearly the opportunity as other methods so if they got less or nothing that would make sense. My guess is that landowners in the system do it more for wildlife control or to have their land better utilized (pro-hunter) than for the few bucks they make from the state.
The state provides liability advantages over just giving out permission directly, so it could be a trade of liability protection for letting a few public hunters on with the hopes WDFW could eventually convince them to move the land into a more accessible version. I know in our area, it's common for land to start in the reservation system and then move into the register to hunt after a year or two with good results and behavior.