My understanding is warranty repairs typically pay less than those paid by the customer, so the dealerships have an incentive to say something is not covered under warranty.
They give them less time. I have a lot of buddys who worked at dealerships and tell awesome stories. On warranty work if the job really takes 5 hours, Ford (and they are not alone here all MFGS are pretty much the same) will pay 3.5 hours. Which leaves the mechanic out high and dry since the dealer typically pays their mechanics flat rate. So what the mechanic will do is find another job to do while he's in there "do it" (aka not do it) so he can be made whole.
This is what happened with my axle seals. I went in with a leaky drivers side, they "changed" the passenger side (wiped both off and pulled it off the rack in less than 30 min). 3k miles later I was back, then they "changed the driver side" (wiped them down and pushed it out the door in under 30 min). 3k miles later when I went back with pictures, had contacted Ford Corporate, and confronted them about defrauding Ford, pencil whipping the repair, and wasting my time. I finally got a loaner car, an apology, a check, and new seals.
BUT it took 4 days because they didn't stock the seals
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As brand ambassadors or "value added resellers", you would figure that the OEM's would realize that if they make the Dealer whole on warranty repairs they would lose the connotation as "Stealerships" and people would happily take their cars there. But this culture has been in place since the 70's, and aint goin nowhere. Instead, the OEMs fight to protect their cars as "intellectual property" that you "lease" under the Digit Millennium Copyright Act to FORCE you to go to the dealer. You couldn't make this shit up.