The wording in 680.74 is such that only the metal parts associated with the water circulation system need to be bonded together. This would rule out the isolated parts like the faucet or water spout that are quite often metal and are connected to non-metallic piping system. Maybe.
If the metal fixture is attached to steel or copper pipe, it's grounded through that pipe. If the metal fixture is attached to plastic pipe, it's not a path to ground, and is also safe. Safer, really; there's no chance of something like that becoming energized, so it's not a potential shock risk from that standpoint, either, and is as safe to the bather as if it were plastic.
Makes perfect sense, actually.