The
FCC are the enforcers of communications law in the USA. While they do not make the laws, they are free to implement regulations as they see fit.
As for "taking" 10 meters, the
FCC can reassign ANY frequency allocation in the USA that is not controlled by the NTIA. If you doubt this just look at that missing chunk of the 220MHz band that USED to be allocated to amateur radio.
As for the ITU ultimately controlling the spectrum through treaties - the ITU had no power to force the broadcasters to relocate on the 40 meter band - they recommended it, the countries signed on to the recommendation, but in the end it was entirely up to the broadcasters to relocate or not.
Out of nothing more than international goodwill, the
FCC should try not to reallocate any band in a way that would cause interference to other users, but they are still free to do so if they see fit.
Every single hz that we have the PRIVILEGE to use is susceptible to being reallocated to another service at any time.