Actually it's both. As well as pharmacy benefit managers, doctors, hospitals, and insurance companies. They all inflate prices. See http://truecostofhealthcare.org/conclusion/ (scroll to the "Where is all this money going" section)

The idea is their pharmacy can negotiate with drug companies. "Give us a good deal on everything or we get generics instead of you for everything we can get elsewhere" (that is but one type of negotiation)