While technically correct, this is almost never enforced.
A dealer has every right to service THEIR own customers before doing anything for someone else's customers, which is what you will be if you didn't buy from them.
If I owned a dealership I would make sure I serviced my customers first whether it's warranty or standard service work. If you bought from me, you would appreciate that fact.
I know hundreds of dealers and this is standard operating practice. I also know hundreds of tractor company people, and this is an expected and accepted practice.
To the original poster: I understand you don't understand why there are different prices for the same tractor, but there are so so many variables that you are probably unaware of. Programs could've been different on those two tractors, a company guy could be willing to give more money off one dealers tractor instead of the other due to age of the tractor, or the volume that one dealer does vs. the other, the dealers location, etc.
It could also come down to this simple fact... Some dealers are willing to sell at cost, and just make the volume bonus. Others are not. I would venture to say that the dealer that is not willing to sell at cost, ie makes money on every sell, will be in business much longer, and be a much better dealer in the long run.