This is a bit of a misinterpretation of Part 15. Part 15 devices bear the "must accept" language to indicate that they enjoy no particular regulatory protection

This is a bit of a misinterpretation of Part 15. Part 15 devices bear the "must accept" language to indicate that they enjoy no particular regulatory protection from interference - the operator of a part 15 device may not compel action on the part of another person because that person's lawful operation is interfering with the part 15 device. Licensed services do enjoy regulatory protection, but that protection doesn't change anything physically - it just means that there is a regulatory remedy when interference is a problem. And one of those remedies is FCC regulations about where other devices can and cannot be used. While radionavigation equipment should be built to high standards for interference rejection, their being a licensed service is in no way a guarantee that other devices won't cause problematic interference. It just means that when other devices do cause interference, FCC enforcement is a possibility.