I'd guess two things in play here. Keeping a truck serviced cost bucks, and some owners cut service a lot of slack and skip much of it due to accelerating fees.
Dealers want trade trucks in good condition, so they can resell high when an owner buys a new truck after XXX many years. So, the manufacturers offer the 'free' service (which we pay for at purchase anyway) and that helps keep the truck in better condition - they hope, and it's probably true.
And, if you have a well serviced truck - with records - the dealer can offer you a little more at trade-in, which motivates an owner to trade earlier than necessary.
I think it's a win-win, an unusual occurrence when buying a new truck. Maybe?