Those thing only used in a few listed cities on the Wikipedia though. And Big cities won't use it, because big cities would have endless amount of Wireless frequencies broadcasting none stopped. And I see your point here though it is not about disturbing the other frequencies in the area you living in. The frequency you are broadcasting in your ard will only limited by the amount of amplifier you've got. If the car is sealed then there is less signal leaking out which will be too weak to reach more than 1-2m when the car is closed. Even in the car the frequency from the N900 FM transmitter can barely reached out to the 0.5m stereo. I had to do a few modification to my car stereo to receive a good signal But it is great that you bring it up so we all know the consequences if anyone abusing the power in their hand.