To describe this as a quirk is a stretch. It is almost certainly the product of lobbying buy US drug companies, and at odds with the VA prescription system, also state funded, which is allowed to negotiate drug prices on behalf of its members.

Back in 2017, I published a note on the US healthcare system, explaining how exactly Medicare had come to be abused by many of the drug companies now complaining. I republish it now, as I am working on updating it, and having the old note on the website will be helpful, when I finish the update. I generally don't like reposts, but this one is useful.

The US Healthcare system is truly extraordinary.  Per capita spending on healthcare is double the levels seen in most other developed countries.  This is in part driven by a very different set up.  Key differences are the private sector has far more freedom to market drugs directly to consumers, and Medicare, the largest buyer of drugs, is prohibited by law from negotiating lower prices.  The result is that the US has higher prices for drugs, and due to the extra spend, also has the most innovative drug market.  It can be argued that the US subsidises drug development for the rest of the world.  However, recent increases in drug prices seem to have been driven by regulatory changes due to the Affordable Care Act (Obamacare), rather than market forces. 

In 2016, total US health expenditures were USD 3.3 trillion.  US citizens directly paid (out of pocket) for USD 350bn, with the remainder paid by third parties.  USD 1.1 trillion was paid for by private health insurance, with Medicare and Medicaid paying USD 1.2 trillion.  Finally, USD528 billion was met by a mixture of other government programs, privately raised funds and charities.     

Of the total USD 3.3 trillion spent on healthcare, USD600 bn was spent on drugs, with half on prescription drugs.  The other half was spent on drugs used in procedures, and not procured via a prescription. While drug spending has doubled since 2007, we have seen a steady increase in the use of generics at the expense of branded drugs.