Rochester, NY - US
I don't know if the horse is dead or not, but let me get a few whacks in just in case...
A crystal is a crystal, when it comes to the tolerances and circuit complexity of RC deisgn. There is no difference between TX and RX crystals. As another user pointed out, there are design specifications other than frequency, but this is only part of the story.
So why won't a TX crystal work in a receiver? It's because the frequency on the label isn't the actual crystal frequency. An RX crystal is at a higher or lower frequency, depending on the design of the radio system. In a single-conversion receiver, the difference is usually 455 kHz. DC receivers add or subtract another 10.7 MHz. Only the TX crystal actually oscillates at the frequency stamped on the case. So if you have a SC channel 30 set, your lane is on 72.390. Your transmitter is operating around 72.390, and your receiver is running at either 71.935 or 72.845. The hobby companies mark the TX and RX so that we don't have to get our calculators out to figure out which crystal goes where.
If you swap the crystals around, your system doesn't work, because the transmitter is now operating on, say, 72.845 (and probably interfering with someone on 52 or 53); while your receiver is listening for someone on 71.935 (which isn't even in the RC band).
It's up to JR or Futaba to decide whether they want to add or subtract 455 kHz, and which load capacitance is best for them, so your best bet is to stick with the original brand or one made for your particular radio. Note also that this frequency difference has nothing to do with transmitter "shift." Shift is an FM property and can be either positive or negative (or both) no matter what kind of receiver you're using.
There's no law against changing crystals. At least, it's not worded like that. Here, we aren't permitted to swap crystals for the above reason, and because changing the frequency of a transmitter may also change the power output, and we're very strictly regulated when it comes to power. If we're already operating at the limit, a few milliwatts could put us over.
The reason Futaba and Horizon make radios with illegal interchangeable crystals is pretty simple. The USA doesn't set the rules for the rest of the world, and as you can see, there are quite a few people flying in Canada and Europe. Americans might be surprised to learn that those Motorola FRS radios you can buy in Wal-Mart require a license to operate at the "5-mile range!" advertised on the package, or that you can buy a CB radio in the USA that transmits over 100 watts despite the legal limit being 12W here (these are made for export to other countries with other limits.) Don't forget we're in a global economy here!
JR radios get around this problem by putting the entire transmitter inside the RF module. You can swap modules because the crystal inside has been tuned and tested to work legally in any transmitter.
It's perfectly legal to put any crystal in a receiver. Put a CB crystal in there if you feel like it. The FCC doesn't restrict the operation of receivers. (There are laws about what you can listen to, though, such as those against eavesdropping on cell phones.)
A ham operator can swap crystals with impunity because his license was issued with the understanding that he is qualified to verify that his transmitter is functioning correctly. If you're licensed to operate on 6m, you can swap crystals in your transmitter. You're also obligated to make sure that you don't cause interference or transmit outside your permission.
I'm a ham operator and a EE, and I know how to test and adjust my E-Flite transmitter, so I'd just as soon save the money and adjust it myself (but you should never assume that just because someone has a degree in electrical engineering they know what they're doing!) I am not the least bit afraid of endangering anyone with my homebrew crystal swap, because I know how to do exactly what the radio tech at Futaba would do. Technically, though, it's illegal because I'm in the States. My ham license does not give me permission to mess with radio equipment on the 72 MHz band. There's nobody going around checking sales receipts to make sure you didn't tamper with your crystals.
But should you modify your radio illegally and then injure someone or damage their property, especially someone with a good lawyer, be prepared to have the book thrown at you. It's heavy.
I hope this clears up a little bit of doubt and uncertainty for someone.-fox