Tire ratings are based on what tires can take. Rim aren't rated for tire pressure stress, though with all the mixing and matching happening these days maybe they should be. Rim makers are in a bind. They have weight constraints in order to be marketable, and can only guess at what combination of tire width and pressure users will subject them to. They make a calculated guess based on the tire/rim width guidelines, but as tire ratings have gone up the potential for too much tire or pressure for the rim becomes a real issue.