Crystal (frequency) changing by an unlicensed end-user on Hitech, JR and Futaba radios operating on 72/75MHz is a violation of FCC rules.
That is the basis of this endless discussion. USA only.
They must be changed and verified by a holder of an FCC 1st. Class Radiotelephone License.
I am not sure if this is necessarily true. The "factory" and service center technicians that install the xtals probably do not personally hold such a ticket. But they are acting under the approval of their employer's established radio business and use the procedures blessed by their firm. This does not necessarily mean that every R/C Tx is verified for spurious emissions (they are not) -- it only means that there is a procedure that is followed.
Crystal changing on any 50 or 53MHz radios/receivers can be done by a licensed radio amateur (technician or higher).
As long as they follow the part 97 restrictions, 6-meter ham R/C'ers (technician class & higher) are allowed to perform the necessary maintenance (including changing xtals).