To be fair, as an attorney, I can tell you that you are correct but the problem is practical in the sense that people don't want to be placed under the burden of having to evidence that a vehicle was properly maintained because if you are a dealer customer, and an issue arises, unless something truly unusual is going on, they are not going to take the position that the customer needs to demonstrate the vehicle was maintained. So, in other words, if there is an issue, there is a very low likelihood that there will be any question about coverage (if the failure is a covered item). On the other hand, if you are not a dealer customer and you stroll in with a factory warranty claim many years after buying the vehicle and no one has seen you since then, they may ask for documentation that the vehicle was maintained. That's not the end of the world either, as owners should keep service records as a matter of prudence.
So I don't think people who use the dealer to avoid potential aggravation when the vehicle is under warranty are at all crazy or foolish. At the same time, if you keep service records, the issue is likely moot for the reasons you describe.
Personally I tend to use the dealers when the cars are new or newish and under warranty, but there are bad dealers out there (like anything else) and I won't be relentlessly upcharged and upsold either. So I keep the records to make sure there are no issues. Another simple thing to do is to keep the truck clean. Drive in with a truck that looks like it has been rolled down a hill, with little in the way of service documentation, and now you are looking for warranty coverage, good luck with that one - I would be skeptical too!
The best of both worlds is when you have a good dealer that doesn't over-charge - that is really handy but unfortunately not that common in my neck of the woods.
Finally, to the validity of the point you are making there is this language from the Mopar extended warranty: "It is your responsibility to properly operate, care for and maintain the Vehicle as prescribed in the owner's manual supplied by the manufacturer. . . . You should retain all maintenance records and receipts to avoid any misunderstanding as to whether or not the maintenance services were performed as required."
Nothing in there about dealer maintenance, but again, folks should keep service records.