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…
That it makes a company liable in case someone repairs their truck and it blows up, since they can't prove that the two were related. Also to consider: the machine blows up after a repair, but it's not related to that.
That it might force companies to publish business secrets: with machine being full of computers nowadays, they might be forced to reveal a proprietary API. Related: someone finds a flaw in the API, uses it to ruin other people's machines, and then the company gets sued for it.
That it forces companies to sell pieces that are not profitable, in case someone wants to repair their machinery. They might have no intentions to get into the spare pieces market, but with a specific law (such as "the company will make available all required pieces for sale, or...") they might be forced to.
Also, money. But those are the kind of arguments you were asking for.