My classmate is in a prestigious business school PhD program - stats / operations research. He told me that they have no teaching duties, and they aren't allowed to do any external internships. The business school's Dean has been furious about letting their PhD students intern in the past and then losing many students early after they land lucrative full-time offers from industry. So apparently, the stipends are increased to make up for the new rule that the students cannot seek internships before finishing their PhDs.
Where does the funding typically come from for business school PhDs?
For reference: United States.