I have actually seen some of these cases.
In all these cases, Apple denied warranty because, even though the devices continued to function, the devices were not repaired/serviced properly (ie. missing screws, screws in the wrong places, missing parts, etc.).
I understand that. But who is in the best position, and who is going to make that determinaton? Do you think a court will let the customer decide if he did damage? The fact is that BOTH sides have self-serving interests there. Don't you think people should be prepared to get into a battle with Apple on this, instead of just walking in and expecting Apple to do the work without question "because that's the law"? That's why this is not the simple legal issue you make it out to be.