Verizon gets a bonus to offer wireless customers to retain existing customers and gain new customers. Especially right before the Christmas shopping season.
Disney gets 50 million customers immediately who may choose to stay and pay after the free trial. Thos 50 million customers also will be free advertising for Disney+ to friends and family who do not have Verizon.
If the partnership is successful, I bet it gets extended to one free year of Disney+ to all Verizon cusomers when they purchase a new phone. If a customer upgrades their phone each year, they can get Disney+ for free each year.
In exchange, Verizon pays Disney a smaller amount for every customer.
It will be no different than Verizon FIOS offering a channel to all of its customers. Verizon pays a small price per customer because they have tens of millions of customers.
Disney gets a large revenue stream that will be consistent based on Verizon's customer base.