US to require COVID Testing for International Flyers

Talleyho69 wrote:
As I understand it, the prevention to crossing borders is not leaving a country, but entering another.

....pertaining to flights only (not ground vehicle crossings), my understanding is that outbound flights (say, from Canada) are (inextricably) tied to filling as much as possible the return flight. ie if no return passengers are allowed to board the return portion of the flight, the airline will cease to fly to that airport.