Proprietary in the computer sense is generally taken to mean closed source or at least not made available to competitors or others. There's nothing to stop any other systems offering this kind of quick start DHCP service.
The only people bent about this are people who have any idea what's going on behind the scenes. In the rare cases where this is a problem, the issue is quickly resolved. If this were a "real" problem, we'd have seen a lot more issues by now. The Apple discussion boards would be lit up with complaints. They're not. This is a non-issue. Only pedants are stuck on the fact that there's no RFC for it.