It's not a corporate mandate, it's a provincial government mandate that exists in the whole of Canada as far as I know (food safety for restaurants being under provincial jurisdiction) and for good reasons, the risks associated with undercooked ground beef aren't worth it to please the small % of clients who would want it.

You want your patties medium? Buy a whole piece of meat, remove the outer layer, ground it and cook it, don't expect restaurants to do that for you.