I don't think I've heard anything so dumb in a very long time (except perhaps the Monte Hall thread)!It's a lever to ensure action is taken. Once Trump is satisfied, he will rescind.
You do know that in free countries, borders are about who you keep out, rather than who you let out, don't you? (The other way about is for totalitarian regimes). Trump is essentially telling Mexico and Canada "close your borders to the people you allow to leave," rather than to those you allow in. And why? Because he's pretty sure even he won't be able to stop the leaks in his own borders that are letting people he doesn't want in.
Jeeeez, somebody should start thinking with that thing on the top of their body!