Carriers have a love/hate relationship with the iPhone. They hate Apple's control (because they want that control for themselves) but love the money and customer-retention having the iPhone on their network brings them. Sprint's willingness to pay damn near all the money in their pockets, and delve into whatever passes for a corporate second mortgage, proves that that point.
AT&T, Verizon, and Sprint sell Android because they want to. They sell iPhone because they have to.