In the US the Magnuson Moss Warranty act explicitly forbids what you are describing

I think something similar applies in many parts of the world, _but_ I also think that the response from the OEMs is basically "fight the warranty claims hard when there is third party service involved, and just honor the warranty with a smile otherwise". Basically malicious compliance is what they have to resort to. And as a car owner I'm lazy and afraid of conflict. So I happily pay X% more for the service to get the no fuss warranty (And the OEM shop is even happier).

	▲	avgDev a day ago \| parent \| next [-]
		In order to deny a warranty claim the US, the manufacturer has to prove the issue has been caused by a modification or servicing. Why would anyone want to service their call at a stealership? The prices are unfair and the quality of techs is awful. Better to find a small reliable shop.
	▲	sidewndr46 a day ago \| parent \| prev [-]
		the couple times I ran into this, I basically just threatened them with legal action & they relented