jayselle,
The reason the transmitter crystals are so easily available here in the states is so that when your original xmtr crystal breaks, you can buy a replacement of the same channel. Not to be able to change it to some other channel. The reason the crystal is so easy to get to is that the radio manufacturers are making transmitters for world-wide use. They're not about to make millions of transmitter cases for international use and a different case with a non-accessible crystal just for the U.S. That would cost them more to tool up two different plastic molds, etc.
The reason we are so concerned with frequency integrity here is that the FCC has given us a specific set of channels to use and they happen to fall between public pager channels and are separated by only 10Khz. That's pretty close together. When someone changes the transmitter crystal and doesn't have their radio retuned to the new freq, it may bleed over to "their" public frequencies and they do monitor the airwaves. Once our hobby equipment starts to interfere with "their" equipment, they have full authority to do whatever they see fit to correct the problem. Will they? I've never seen it happen. I'm more worried of being shot down by a "bleeding" transmitter that someone carelessly modified. I now take a frequency scanner with me everytime I fly at my field. I shoot the airwaves, and if there is a hit on my channel, I will change the module on my transmitter to a clear channel.

Jesse