We've had two Tesla Model 3's since 2018. One has been flawless (Model 3 Performance) but my wife's car threw an error message saying there was a problem with the driver's seatbelt tensioning system. This is a pyrotechnic device that tightens the seatbelt automatically in the first few milliseconds of an impact serious enough to activate the airbags and is designed to reduce potential injury from airbag deployment.
In this case, Tesla didn't warn us of any diagnostic fee because the car itself notified us of the issue. Even if an investigation revealed it was a false alarm, Tesla would cover the resetting of the error code as a warranty item so there is no ambiguity.
The diagnostic fee might come into play if the owner reports an aggravating rattle. This may or may not be a warranty item depending upon what's causing the rattle. I used to own a Volvo and one of the members of one of the Volvo owner's forums was complaining that Volvo wanted to charge an hourly rate to track down the rattle. The dealer said if it was a warranty item the owner would not need to pay the charge but, if it was non-warranty, then the owner would be on the hook for diagnosing the rattle. This really pissed off the owner and he claimed it was illegal, yadda, yadda, yadda. In the end, he had to agree to the possible charge to get the dealership to diagnose the rattle. It turns out the rattle was caused by some sunglasses the owner had left in one of the compartments! They charged him for 1/4 hour and retrieved his sunglasses. The owner was really embarrassed about the huge stink he had made over what turned out to be his own forgetfulness.
As you can see, there is a valid reason why this type of diagnosis fee does not violate warranty laws nor common sense. The dealers need to protect themselves against unreasonable demands. I mean, why should the dealership pay salaries, benefits, employee taxes, advanced training for their technicians, etc. just to take care of forgetful buyers? They would have to pass those costs onto all their customers, even the non-forgetful ones.
At some point, people need to take responsibility for their own actions! It's completely legitimate for shops to pass on non-warranty diagnosis work to their customers. Only if it turns out to be a fault of the car does the diagnosis fall under warranty coverage.