Why would drug companies allow it? When a lot of Americans were filling their prescriptions in Canada, the drug companies simply applied quotas to their shipments to Canada. They know how big the markets are, so why would they ship 500% of estimated demand to Canada knowing it would flow back into the US?

As a result, a lot of pharmacies (and some gov'ts) restricted sales to out-of-country patients to prevent shortages in their own countries.