In my present facility, I do not require my boarders to use my "preferred" vet, farrier, or "in-house" trainer. I use this as a "selling tool."
Most barns do require that you use their vet, and farrier. It is also easier to have one service provider for your barn, if your records need up dating. Worming, is also a means of income for a barn.
I will admit, when I first started to board horses, in my younger "starving years," my horses were trimmed free, discounted, and vets would do the same (personal horses) on minor services, if already on call with another boarder. Most barns that bill your "services" will pad the discounted bill and add handling fees. Trainers will also do this.