I'll give you one guess on which industry lobbied for that restriction.
1. Medicare only pays "average selling price" which includes all discounts offered to physicians
2. Medicare demands pharma pay 50% of the donut hole gap which is several billion each year
3. Medicaid automatically gets a 23% discount on all drugs
4. DOD, VA and other gov't organizations do negotiation drug prices and they get a 24% discount on top of that
There is plenty of price negotiations on the gov't side.