The Wall Street Journal, quoted in a MacRumors comment:

Early on, both sides determined it would be a bad idea for Apple to offer its own cellphone service, leasing access to Cingular’s network. Even though Virgin Mobile USA and other startup cellphone operators were using that method with some success, Mr. Jobs was cautious. He viewed the cellphone business as an unforgiving one, where carriers are blamed for network problems and overwhelmed by customer complaints.

Taking credit for the iPhone, and letting Cingular take the blame for anything that goes wrong? Why, yes, it makes perfect sense. (But then why doesn’t Apple migrate .Mac to another provider?)