Depends how you look at it. Many microprocessor devices that tune to radio signals will be using a PLL (phase locked loop) which will include genetating a signal within the unit equal to that frequency which it's trying to recieve. In normal circumstances this would be fine, but since the unit isn't FAA/CAA approved there are no gurantees that the unit may not malfunction and begin transmitting energy on these frequencies. That would be bad, especially if the a/c was actually using GPS to navigate as the signal may be lost.
Same reason why you shouldn't use a handheld AirBand reciever, FM reciever or as the offical line goes - "any device capable of transmitting or recieving radio frequencies".
My mate was once asked to turn his GPS reciever off on a trans-atlantic. He was told it could interfer with the planes systems. And YES it could, but (arguably) so could a CD player with enough malfunctions.