If they were only reimbursed for exactly the cost of the drug, they'd lose money on expensive drugs because of inventory costs. Inventory costs include interest on money for the time they're holding it, damage and spoilage, security, theft, insurance, etc.

6% is probably a reasonable guess at the average costs. Large, high-volume operations can make a profit by keeping their costs below 6%, and that may create a wrong incentive. But the right answer isn't to change 6% to 0%, because nobody will want to handle expensive drugs.