It is none of their business, but they desperately want a piece of the pie so they brand their phones (and probably sell them at a small discount - this is the

It is none of their business, but they desperately want a piece of the pie so they brand their phones (and probably sell them at a small discount - this is the value proposition here). Branding also results in shifting responsibilities from OEM manufacturer to carrier with regard to testing and stuff. Also, plenty ISPs actually do sell branded wireless routers with branded firmware, so it's not uncommon practice in other neighboring industries.