It has to do with how tightly packed the frequencies are in the United States. There are many regulated frequencies that are closely spaced with the ones we use for our RC equipment. If a radio is mistuned it can interfer with commercial operations it can cause them a loss of money. For instance, television channel 4 is very close to our frequencies. If it weren't for this rule about tuning we probably wouldn't be allowed to even use the 72 Mhz frequency range. This would leave a lot of pilots out of luck.

Ken