I honestly don't know if the rules are different on 75Mhz, they very well might be.

But it IS against FCC regs to change crystals on 72Mhz. You can change the module if the radio has one, but just a crystal is not legal.

Just because it's common practice to chang the crystals at races doesn't mean it's leagl. I've seen lots and lots of things done as common practice that were against some rule or another.

It's also worth noting that if a car has a couple of radio hits, it's usually not as bad as with an airplane. Not that a car can't hurt or kill someone, I've seen enough nitro cars get airborne at head-level to know someone could be hurt by one. But with a car, if you have a suspect radio while running and start having intermitent control, you can kill the engine and just stop. Airplanes don't have that option, and they can travel a lot farther to reach out and touch someone. So, I'd personally be willing to do more stuff with the radio with a car than I would with my planes.

You're right about the crystal sales being inconsistant. So is having the crystals right there where you can easily change them. Believe it or not, to pass the FCC acceptance, some radio companies claim to glue the crystals in so they aren't "user replaceable". The "reason" crystals are available is so that people who are licensed to work on radios can get them to make the change for you. You don't have to be the factory service center, just have the right license to legally change the crystal.

And, like I said, outside of the US, the rules are different. So it makes sense for the manufacturers to just make the same product and ship the same parts everywhere.

And, frankly, the FCC has better things to do than chase around RC guys swapping crystals. So you can be pretty sure the FCC isn't going to actually enforce the rule anytime soon.

If you have an accident, though, you should at least know what you did is not techically OK.

But, as you guessed, most of it is a bunch of nonsense, at least with MOST current transmitters. I won't say "all" however, there could be some that won't take kindly to a crystal swap.

Personally, if I wanted to change my TX to another channel, I'd just do it as well. But I'd make the change a small one, just in case. And I'd carefully range check the radio before and after the change, to make sure the range didn't change. If you notice a drop in range, you know why. (But, since I own a Polk Traker synth radio, I can just pick a channel, no crystals. Hitec and others have this kind of thing as well. I suspect eventually crystals will be a thing of the past).