In the US there are commercial users sandwhiched between the "channels" we use. You can change 50mhz crystals all you want provided you have a ham ticket to operate there as they are the only ones on the band. Honestly the FCC doesn't care about you shooting airplanes down (though your insurance carrier might), they care about interfering with the commercial (read that: PAYING) users of the band.

That situation is unique here, hence the rules. They sell the crystals because there are plenty of shops with techs trained to check the tuning.