You have to consider that an Airplane is a complex device operating in 3 dimensions and is not really constrained in any of those dimensions. (Car car is really just operatoes in 2 dimensions unless it goes airborne and is then uncontrolled and only ballistic) As such, the idea is that safety of the occupants of the plane, those who might be in nearby aircraft and those on the ground would require that in cases where some device might interfere it would better if the possible interfering device be disabled.
And it is not always that your device would directly interfere, but it would be possible for several devices to interact and by heterodyning to produce signals that could be problematic.
Radios do not just use the receive frequency, but they internally heterodyne with some Intermediate Frequency (mixers and IF), often over several steps to get to the decoded and usable signal. A common IF frequency in consumer products is 10.7 Mhz. SO - if you were wanting to recieve a signal at 120.7 Mhz, you would mix it with an Intermediate Frequency of 110 Mhz, or 131.4 Mhz. And depending on what other signals are out there a strong signal on 99.3 Mhz or 142.1 Mhz could overload the desired signal (interfere) and keep you from receiving the desired information.
For recent news about such a problem lookup GPS & LightSquared. LightSquared wants to put a terrestrial service that uses frequencies close to the GPS frequencies and thinks that everyone should upgrade their GPS so LightSquared won't have to worry about interfering. Close Strong signals overloading a reciever that is looking for a distant weak signal renderring the GPS reciever unusable.
SO the issue for Aircraft is this - would you rather turn off your electronic device, pay for testing to see if there is a possible problem, or just take your chances that it will work OK? The FAA opted for the safest method, shut things off and they CAN'T interfere...