There is no legal requirement to proof an rifle after threading
None
Think about it.
How many are done and pass through the hands of reputable dealers
if there was any sniff of litigation coming their way they would not accept P/X, aftermarket threaded rifles.
In order to successfully prove that the integrity has been compromised you would have to demonstrate a failure AND more importantly test the theory in a court to set that in law....rather than what the proof houses would have you believe.
It is in the proof house's interest to have all threading jobs proofed
What I find very dodgy indeed is the practice of proof houses stamping the underside of muzzles on new builds to somehow attempt to enforce this should at some point in the future the barrel be threaded and the muzzle "proof" mark be removed.
This is not a legal requirement, is arguably done without consent, has suddenly become the norm but no-one complains