Then JD should have the owner sign an agreement that says JD not responsible for repairs/accidents due to non-JD service.
They don't need any such agreement. They already have a federal law that says this: the Magnusson-Moss Warranty Act of 1975. It basically says that for repairing or maintaining your car (or other consumer good), you're allowed to use 3rd-party repair parts and have an independent mechanic (or yourself) do it, unless the manufacturer does all this stuff for free for you, and that they can't "void" your warranty because of this, but that they *can* refuse warranty claims that are actually caused by the non-OEM part or repair.
Maybe there needs to be a law.
There is, the one I just pointed out. Somehow, after 42 years, everyone's forgotten about it even though almost everyone in America drives a car and is affected by it. Honestly, even on car-related message boards, full of car enthusiasts, I see this come up constantly, with people asking "will my warranty be void if I don't take my car to the dealer to change the oil?"