* That modern tractors are complex enough systems that only official techs should be working on them.
* That if they allow third party mods, they can't guarantee regulatory compliance with emissions.
* That they have proprietary secrets that would need to be divulged of the systems were more open, and they just wouldn't have bothered in the first place.
* That their pricing model is dependent on recouping initial capital expense with service contracts.
I personally don't buy these arguments, or at least don't buy that they lead to a better world than the alternative, but those are what I've heard.
Regulatory compliance is an interesting argument. I can imagine many cases where a device is legal only because the firmware implements certain restrictions that the user might desire to remove but should not.
Off the top of my head:
1. the emissions example that you make. The VW dieselgate is essentially about this; the users may want to switch to a 'more dirty' firmware because that gets better performance or mileage; but the vehicle inspection system (in countries where that is a thing) wants to ensure that vehicles with such modifications are not allowed to drive.
2. digital radios that might technically be able to receive and transmit over a wider frequency range, but are legal to use or sell only because they have restricted the 'forbidden' ranges, limited the maximum transmitting power to the level that doesn't require a permit, etc.
3. scooters and other vehicles which are technically more capable, but are sold in a certification/tax regime as "speed less than X", which is enforced in firmware; and modifying it to remove that restriction may be illegal.
4. drones obeying a 'no-fly' zone blacklist and refusing to fly near airports - again, a firmware restriction that some users might plausibly want to remove.
Those arguments don't work in the automotive market. Modern cars are just as complex, yet anyone is allowed to change their own oil or do anything they want really (though they can get frustrated by anything requiring a proprietary scan tool). And car owners are allowed to mode their cars however they want as long as they don't run afoul of emissions laws (yet many of them do anyway, because enforcement is poor).
* That modern tractors are complex enough systems that only official techs should be working on them.
* That if they allow third party mods, they can't guarantee regulatory compliance with emissions.
* That they have proprietary secrets that would need to be divulged of the systems were more open, and they just wouldn't have bothered in the first place.
* That their pricing model is dependent on recouping initial capital expense with service contracts.
I personally don't buy these arguments, or at least don't buy that they lead to a better world than the alternative, but those are what I've heard.