It is not quite as sketchy as it sounds, but it’s still bad.
The medications in question are specially medications such as biologics that have unusual requirements like needing to be administered via infusion in a clinic by a nurse, or ultra expensive medications that are 10,000$+ per dose that insurance companies don’t want mishandled and need to be discarded. All stuff that you wouldn’t normally be able to stock at your corner pharmacy.
The pharmacists need special training on these medications, by limiting it to a particular chain, in theory they can ensure better care (which in theory saves insurance companies on claims on medications that were improperly delivered.).
But still, there’re several specialty pharmacy chains that deal with these medications, patients should have the right to choose where they get their medication.