As an engineer in a big multi-national I also see similar things going on in our company.
They try to prevent untrained/unauthorized technicians from doing what we call "low-level" maintenance even though our equipment might be of vital importance of that buyer.
In our company this is not necessary bad intent towards the customer, but more a way of protection our own business because selling only gives you 1 paycheck, service gives you hundreds in the course of years.
Our machines are pretty comparable in complexity to modern tractors I believe as years of research and development have made it so they are of higher quality for the customer. This does not immediately relate to longer life times of our products but does improve on requirements because of new industry, government & environmental standards.
But it also makes it harder to do a correct maintenance if you don't know the complete working of the machines.
Anyways, I don't want to justify John Deer's way of working, or any other car manufacturer ( because that seems to be the case here in Europe), but I do understand their position better.
The customer should be informed when they buy a product that their new product can only be maintained by the approved technicians, there for the EULA probably that has been forced onto the farmers.
I also don't know how the market competition is for farming vehicles in USA, Europe or the rest of the world.
And I think that part should be fixed then, if there is no (reasonable) competition/alternative for the farmers then there is a problem there.