Magnuson-Moss Act: http://en.wikipedia.org/wiki/Magnuson%E2%80%93Moss_Warranty_Act
This is what protects you. But I agree with EnterTheDragon, you will have to hire an attorney to fight them if it gets to that. Having said that, SOA will most likely not get to that level since it costs them thousands of dollars to go to court. Just use common sense, if the aftermarket part has any chance of causing the failure of any part, install it with the understanding that you may have to take responsibility. You gotta pay to play.