Ever had a problem when swapping crystals?
Some technical information.
Crystals are not absolutely accurate frequency references. The actual frequency of the transmitter depends on how accurately the crystal was made, and also on the tolerance of certain component in the transmitter.
In and ideal world, the crystals for each channel would be identical, and the components in the transmitter would be identical as well.
However, they are not. So, when you plug a crystal into a transmitter, it will operate at some frequency which is close to the nominal frequency for the channel. The FCC requires that the transmitter be within 20 ppm (parts per million) of the nominal frequency. At 72 MHz, 20 ppm is 1.4 kHz. This has to be acheived over the full battery voltage and ambient temperature range. Most of the time the transmitter will be within 20 ppm with a randomly chosen crystal, but not always. While it's possible to build transmitters and crystals that meet this specification without adjustment, it requires the use of precision components, and some element of temperature compensation.
While the necessary precision components may not make much difference to the cost of a high-end transmitter, they will affect the margin of a $99 radio noticably.
It's cheaper to pay a factory worker to spend 30 s adjusting each transmitter as it comes off the line than build a bullet-proof design. If the radio were manufactured in the US, the cost trade off would most likely swing the other way.
It is possible for this to be done. Muliplex have apparently been certified with user-changable crystals. To do this, my guess is that they've explained how the tolerances of the compenents in their oscillators stack up, and demonstrated with a sample of production transmitters and crystals that the frequency is always within specification.
If Futabe, JR, etc. chose to , they could go through this process.
The FCC is not concerned about whether we shoot each other down, or that the transmitters bleed over into the next RC channels, or are so far off frequency that the model flies out of range and crashes. They are concerned that the transmitters don't interfere with the license fee paying users of the 72 MHz band who occupy the frequencies between the RC channels. There is only a 10 kHz frequency difference, so with a 10 kHz bandwidth, an RC transmitter does not have to be far off frequency for its sidebands to interfere with the paying users.
Synthesizers use a single crystal oscillator as a frequency reference for all output frequencies. Provided the reference is within 20 ppm, any channel on the output will be withing 20 ppm. The reference will be adjusted to the exact frequency required as part of the manufacturing process.