"It would be no different than GM being sued because..."
Sorry, the right analogy would be if GM forced you to use its own fuel, oil, tires, sparks, windshield wipers, mats, phone, USB storage, music, etc. - making them 30% more expensive. Nobody is asking Apple to be able to install their own processor, or memory - and keep the warranty.
Nor anything or anyone would stop Apple to run an "Apple tested and approved" program for developers and users who want to be "sure" the apps aren't malicious.