I have also found some of the regulations quoted on a Futaba FAQ:
http://www.futabarc.com/faq/service-faq.html#q2
They quote yet another section of the FCC regulations than any of those mentioned by JohnW.
All of which begs the question, why does my Futaba radio have the crystal on the front, as easily removable or changeable as any switch. I can imagine it's due to the radios being sold overseas, where it is legal for the end-user to change transmitter crystals.
At any rate, thanks for all the specifics. While I knew it was historically not permitted to change crystals here in the US, I thought perhaps the rules had recently changed.