You got to pay to play just like anything else. The thing is the dealer has to prove the aftermarket part caused the failure. So if you tune a PCM they can't say thats the reason why your fuel pump quit, or your radio isn't working anymore.
Any tune is ulimately visible to GM. Period.
I'm not sure if the people who advertise "stealth tunes" have thought about legal issues. I'm no expert but it seems not to be a good idea to deliberately cheat your warranty contract. It seems like it could constitute fraud