Now you're getting into the diplomacy area.
Obviously, the TSA can not force other countries to comply with TSA regulations. What can happen is that the TSA determines that the policies are not being followed and therefore the foreign-originating flights may not land or disembark the passengers. That's the TSA's real weapon.
Using the Northwest/Delta flight as an example, if the TSA had determined that flight didn't pass the security requirements, then all passengers and crew would be rescreened on arrival. If it was a recurring problem, then that flight/airline/point of origin would be denied entry into US airspace. There are a bunch of foreign airlines that are prohibited from operating in the US due to security and safety issues.