It may depend on the size of your barn, or how your barn was established. This is my 4th facility, in 4 different states. (NM, TX, CA, OK) I can tell you that it is very common and expected. It also extends to trainers.
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.