In most situations, even that is giving too much power to the manufacturer. It’s fair for them to flash the original software as part of any diagnostic or service process, but not fair to refuse to repair or replace a product that actually has a hardware defect just because the owner put different software on it.
Locked bootloader for warranty coverage: totally fine
Refusal for owner to unlock and void warranty: not fine.
In most situations, even that is giving too much power to the manufacturer. It’s fair for them to flash the original software as part of any diagnostic or service process, but not fair to refuse to repair or replace a product that actually has a hardware defect just because the owner put different software on it.