Except for the fact that being on the no-fly list means you can't even enter US airspace regardless of your origin and destination and many (most?) flights from Canada still pass through US airspace. Same thing with most Mexican flights to Europe.

Here are some eye-opening stories:

http://papersplease.org/wp/2011/02/16/british-man-marooned-i...

http://papersplease.org/wp/2011/05/25/us-intervenes-to-block...

http://papersplease.org/wp/2011/07/25/mexico-barcelona-fligh...

http://papersplease.org/wp/2009/05/16/air-france-passenger-d...

http://papersplease.org/wp/2010/06/07/another-paris-mexico-f...