Good point. I forgot to mention earlier, but in bma's example of an SSD upgrade and a flickering screen, I disagree that that would be clear customer damage. Clear customer damage would be an SSD upgrade, flickering screen AND scratches, broken components or other physical damage on the GPU, cable or port. The onus would be on Apple to prove the customer caused damage directly if it was Apple that wanted to void the purchase contract.Essentially, if you live where there is such a legal wording in the law, you can modify the equipment to suit your needs as fair use (computers, cars, etc), so long as when you eventually claim warranty, your actions CAN NOT BE PROVEN by the manufacturer to have caused the issue at hand.
My 2006 imac had developed screen problems which is a common fault of that machine's particular gpu. As I mentioned earlier, there was no problem with a warranty replacement. My SSD upgrade had clearly not interfered with the faulty hardware.