My guess is UA makes more money on the island hopper than on the nonstop, due to the monopoly stranglehold they have on the islands in-between. Since the island hopper is often full, they perhaps try to make space by booking people on the island hopper on the nonstop. Of course, that doesn't help you if the island hopper what you intentionally paid. They should have a system that would ask you to move to the nonstop for free, instead of just moving you for no reason.