There's a difference between liability in causing accidents - and support/warranty.

Think about it: some guy changes the suspension on his car, making it super tight, puts racing wheels on - and then claims an issue on 'warranty' when something happens due to his modifications?

It's definitely within reason to forfeit a warranty if someone hacks up the product with their customizations.

There's no way an auto-manufacturer can provide warranties for parts, which depend on other parts - that have been altered.