The government simply has a max price it will pay for certain drugs. The drug companies can sell them at that price or they won't. How is this coercive?

The money the government will use to buy the drugs is acquired from taxation and deficit spending. Taxation is coercive and deficit spending demonstrates that the government does not even limit itself to tax revenues when it exchanges on the market. In short, it's entire existence on the market is supported through coercion.

This upsets price equilibrium. The intersection of supply and demand curves are predicated on voluntary exchange. Submarginal buyers don't buy when the price is at a certain level. The government is never a submarginal buyer because it will either increase taxes or deficits to get what it wants.

The appearance of voluntary exchange in this one tiny facet of the larger market is just that...an appearance. The underlying reality is much different.