Bullshit, warranties is the way to keep manufacturers honest : if it only has minimum legal manufacturer warranty, you know it's crap.

And it should be illegal to void warranty for rooting.

My understanding is that (precedent aside) in the US the law clearly says that the manufacturer has to prove that you managed the product under warranty.

That is those "warranty void" stickers are not enforceable (under that specific law) by themselves, they need to argue that the user actually damaged the product.