You're assuming that everything not mandatory is prohibited. If the device is required to provide every service with the flag, every service gets the flag, even if it contains no adult content or adult content that the user agent could display or not without the service having a way to know about it.

The service would then have to deduce the information instead of getting it explicitly and may be able to do that some of the time instead of all of the time, which is an improvement. And then people can work on anti-fingerprinting technologies with the premise that if they succeed it actually does something, instead of the information being required by law to leak to the service.