Surely signals aren't that strong!
I would have thought that wireless mics would operate at such low power and would therefore be fully contained by the 'almost' faraday cage status of most buildings.
If a building has normal wiring, then in most instances rf won't penetrate out of the building at sufficiently low power, but would be strong enough inside of the building to be functional.
Seems they're erring on the side of caution, when there's little need.