While that may be true by the letter of the law....reality is very different. The law states that they must prove the modification was the cause of the component failure. But the court system says they are innocent until proven guilty.
You modify your vehicle, then take it in for a [legit] warranty repair. They deny the warranty claim. You refuse to pay for it. You argue back and forth, but in the end, they still refuse the claim....and they have your vehicle (at worst), or you have a broken vehicle. What else do you have? Grounds for a lawsuit? Sure.
Hire a lawyer, jump through the hoops, pay for a rental car for the next 18 months while the law suit works its way through the system. They just keep filing motions, which drives up the $$$ you owe to your lawyer (because you have to respond). This keeps on going until either you or the dealership are broke. Wanna guess who goes broke first? Once you are broke and don't have a lawyer, you miss a hearing or two and the case gets tossed...now you are broke and still don't have your vehicle fixed.
Welcome to reality...