If I swap the parts myself, I either get the new parts from apple themselves (in which case it IS their fault if something goes wrong or they can blame it on me for wrongly assembling it; e.g. "You were holding it wrong")

If I go to a shop or a friend to get my phone repaired and they replace parts, this would be different. If the device malfunctions I would obviously blame the guy who last fiddled with it first. Look at car repair shops, household appliances etc...

In the case of batteries its even more simple. If its an apple battery, its obviously apples fault if the device catches fire. Because either the battery was bad or the manuals were. If the swap is so terribly difficult that many users will damage the phone while swapping batteries, it's also apples fault, but they will (as usual) deny any claims so no harm done.

If it's a third-party battery, I can't see how the blame would fall to apple.

All in all, the reputation argument is on them. I don't see how e.g. Ford can be blamed for a car where the independently repaired brakes didn't work and caused an accident.

I never seems this argument when you install after market car parts and install the parts yourself or your neighbor or some random car service, if you changed your lightbolb in the car and the engine breaks 1 month later you can't void my engine warranty. When I install a cheap battery, the pone burns and I bring it to warranty then the warranty people can photo the phone, show the problem, show the faulty battery and I am at fault.

If you are concern that evil people are out to get Apple, those evil people have many other ways to do it, there is no good reason to screw 99,99% of your user because some random guy install a cheap battery and then puts a photo and rand on the internet, that does nothing, there was a need for thousands of people to complain about the keyboard issues before most(but not all) of Apple fans believed that Apple could make a mistake(so a few rants won't have any effect).