I think it would be interesting to go into that shop with a nice vintage bike that hasn't been cleaned for a while... Or perhaps artificially dirtied up...

Of course, if I were to do something like that, and then find the dishonesty, then there is the question of what to do about it.

It would be a tedious process to prove the issue, just as it would for a car mechanic or home repair person... It has to be easy to get away with this sort of crap, and low risk if caught or people wouldn't do it.