Honest question: what would be the rationale to prevent farmers from repairing their machinery themselves instead of through the manufacturer (in this case, Deere)? To my mind, from the point of view of the "user" (i.e. the farmer), that can be seen as somewhat monopolistic. I tried to find similarities between this situation and that of Apple not letting users tamper with their hardware, but it could be argued that…
I'm for right-to-repair, but I'll give it a shot in defense of manufacturers. A lot of people assume it's just "evil-corp" greed. All of these reasons are money based, but not necessarily malicious. 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…
In both the other cases, people will happily reverse engineer the documentation and create secondary supply chains. The company has to go out of their way to shut both of those down. Not to mention hardware-DRM measures.