Actually, they can't.
Bluetooth has a maximum range of around 10m, and a latency you'll notice.
Plus it sits in 2.4GHz like everybody and their dog - I've dropped several calls due to somebody driving past me also on their Bluetooth headset.
EL Reg - You're wrong, it's not illegal.
Wireless microphones in the US have been using TV "Whitespace" for a very long time - that includes TV stations and Broadway productions.
The rules just meant that you had to be aware of other users - it didn't really matter, because a TV transmitter powerful enough for a TV to receive will almost always stomp on a microphone.
Finally, according to http://www.shure.com/proaudio/products/us_pro_ea_dtv
" the FCC has ruled that wireless microphones may continue to use all presently unoccupied TV channels until the end of the DTV transition"
- It's not entirely clear what'll happen after the transition though.