Legal if the building is constructed such that the radio signals required cannot get in or out due to the materials used, like a giant farraday cage.

The FCC does not have the authority to stop you from doing that.

Their authority is in the area of regulating who can USE the airwaves.... they regulate who can broadcast, and how.

And in this case, they are saying that building a device that actively transmits with the intention of preventing other devices from working is illegal.

Private property is irrelevant - the airwaves are public.