> Liability: If a customer repairs a machine improperly and it causes more problems, or someone gets hurt, it can paint the manufacturer in a bad light. Many farmers know how to operate a tractor safely, but might not know how to disassemble and repair one safely.

That is not a reason to make repairs illegal. Authoritarianism does not solve problems. If property means anything, then it certainly should include the right to use it "incorrectly", though obviously with liability for any damages you cause others.