Mmmm. I just don't know why they didn't do that. It doesn't seem to make a lot of sense, really, and I kind of suspect it may bite them in the arse when they want to release a new magic item book, and pretty much every sane way of having a magic belt that "does X or Y" has been expressed. I suppose that'll be the book that says precisely what you're saying, though
There's something hugely perverse about that, to me. Like, WHHHHHHHHHY? Like, it's multi-stage idiocy, the way you describe it. I don't see any logic to it beyond "Gosh I feel like maybe we should annoy a lot of players for no clear reason!". Rings aren't cool. Maybe that's the logic. Rings aren't cool so we'll force them to be cool by making them overpowered? Pffft. Then apparently they've made them SO overpowering that it's impossible to have a balanced game if you wear two? Seems a bit "out of whack" to me.