But the only reason "price by fiat" works is because the Canadian government has the authority to set a price ceiling for a huge number of buyers. UHC doesn't have the same market power, because it's competing with other insurers who can potentially cover the same people. If Canada decides that a drug is worth no more than $X, there is no way to sell to Canadians without agreeing to that price. If UHC decides a drug is worth no more than $X, in the short term you can't sell to UHC customers without agreeing to the price, but in the long term you can exert leverage over UHC because people might switch to other insurers who cover more drugs.

You need to know the history: the pharma companies wanted the current system in Canada, because the previous system was compulsory licensing: ie: anyone could manufacture a patented drug as long as they paid a royalty.

It was a lot like playing songs on the radio: nobody could stop you as long as you paid.

Then the system was switched to give the pharma companies total control of the distribution of their patented molecules.

That sounded a lot like nationwide monopsony power. Compare to "Wal-Mart simply decides what TVs are worth and then tells the manufacturers. If the TV manufacturers don't like it, they can't sell them in Wal-Mart."

Perhaps United Healthcare cannot negotiate as much because it's bound by regulation, there's a large number of drugs that it cannot realistically stop covering, and the drug companies know it?

Monopsony is “one buyer”. The group doing the price regulation in Canada isn’t buying anything.

On practical terms, they are acting as a monopsony. They are negotiating with the manufacturers. If the price they set is too low, the manufacturers will indeed stop selling in Canada.