That's a plausible story for drugs which already exist. The problem, though, is that they wouldn't bother researching new drugs in the first place if it weren't

That's a plausible story for drugs which already exist. The problem, though, is that they wouldn't bother researching new drugs in the first place if it weren't for the prospect of those "absurd profits" in the USA, because that research would never pay off. Once they have the drug developed it makes sense to sell it wherever they can, at whatever price they can get (above COGS and overhead, of course), as profit is profit—so long as it doesn't cannibalize their much more profitable sales in the USA. If the USA were to suddenly allow retailers to import drugs at scale from other countries to reduce costs then those countries would find their supply of cheaper drugs drying up. They could either accept prices on par with those in the USA, thus eliminating the arbitrage, or simply go without.