The real reason why European countries (and Canada) pay less is because they legislate discounts. They give the drug companies no other options. I would call it arm-twisting more than negotiating.
And yes, Medicare doesn't negotiate, it legislates discounts. Medicare pays no more than the Average Selling Price for a drug (average price across all sales). It also legislated that the manufacturers pay a discount to cover the donut hole for Medicare Part-D.