The agreement is likely between the customer and Verizon, not the roaming partner.

The other thing is some of these rural providers don't actually sell local subscriptions, their business model is putting up the capital for the cell sites and soaking in the roaming revenue from multiple national carriers.

While this example does sell wireless service themselves, there's a company Pioneer Wireless in Kansas and Oklahoma. They are seperate from Verizon, yet to a VZ subscriber, it looks like native LTE coverage.