Putting PPF on could be used as an easy out to void a paint warranty. PPF and wraps have caused paint damage to cars in the past, so it's not unimaginable. Yes, the manufacturer would have to show that the PPF caused the issue if you were to take it to court. The issue is most car manufacturers have endless money for legal counsel. By the time a consumer would be able to take a car manufacturer to court, it wouldn't be worth it from a time or financial stand point.
If you ever have a warranty issue in process, don't do anything stupid that could give the manufacturer an out before they fix it. It's like if you complain about engine noise on a new car and tune it before the dealership looks at it. Of course they're going to say you put a tune on the car, engine warranty is voided...bye.