I recall an engineer's report attempting to measure spurious RF emissions from a Garmin GPS, and basically showed it was very clean - far more noise from a laptop computer.

I used to have no problems using a GPS on board a plane, it wasn't until they became commonplace that airlines rules against them. As you say, it's lazyness. The only justification might be that a battery could overheat and catch fire but so far it's only been laptops that have had this problem but of course banning use of those would kill business travel.