While I can understand not letting someone use my barrel as a guinea pig to learn barrel threading, it's really not a difficult process. Any competent machinist should be able to do it.
I bought my first suppressor 35 years ago. Spent a lot of time on the range as a student, trainer, RSO.
I have seen several cans blown into orbit and end cap strikes because the threads were cut by "competent machinists."
And I can't count the number of times I have seen perfect .5x28 centerfire rifle threads cut on a rimfire rifle. The machinist cut the threads just like the shooter asked. Except the shooter did not understand the difference between the two.
What about the rimfire threads for the cans that use an o-ring? How much of a gap do you need for the o-ring, and where does it sit?
And if "competent machinists" cut shoulder reliefs correctly, then why do so many people have to use beveled spacers, or should?
If all the machinists were competent, then why do so many suppressor companies make user replaceable threaded end caps?
There are very good reasons that there are shops that only cut barrel threads.