This doesn't just prevent fixing - it prevents modification. If I want to change something about my car or motorcycle or tractor or airplane I should damn well

This doesn't just prevent fixing - it prevents modification. If I want to change something about my car or motorcycle or tractor or airplane I should damn well be able to (assuming the modifications comply with all laws, which is not the issue here). Other comments say that these tractors are always purchased with a loan, in which case the loan provider (and insurance companies) can make stipulations about the usage and maintenance of the tractors. And perhaps John Deere can help enforce those. But for them to just put a unilateral blanket ban on modifying their equipment is wrong. If they have a monopoly it is an abuse of power and should be punished; if they don't then the market will fix the issue itself, for example via farmers complaining and then not buying from them any more.