I've never heard of any Canadian student not being able to attend an American college due to a visa issue.
I think we're actually in agreement here. nebbalish is certainly allowed to apply to other schools but if s/he is admitted by Columbia via ED s/he is obligated to withdraw all of his/her other applications or not file them in the first place. If s/he has other offers at that time s/he is obligated to turn them down. You clearly understand this.
However, nebbalish intends on applying to another school and attending that school instead of Columbia if s/he is admitted to both. This is basically treating ED like RD, as if there was no ED agreement. S/he wants to see both results and if s/he is admitted to both, s/he will weigh the offers and decide from there. That is clearly and unarguably forsaking the ED agreement. It's taking advantage of Columbia's lower selectivity in ED (they assume they'll get 100% yield on offers so it's less random than RD) while not giving up the flexibility in selection that's required to get it.