I just don't see why someone should be mockery free just because they're french.
I think it has to do with a trio of facts: a) The French are a minority in this country that have a long history of being marginalized and treated unfairly by English Canada and b) The fun that was made was not of poutine, Celine Dion, the weather or some other mundane franco-Canadian thing - which is fair game, but rather their ethnic origin and their right to speak their language (probably latent American hatred of all things Freedom...erm...French, post Iraq), and c) It was done by an outsider.
Again, going to Victoria, Toronto etc and making fun of the group in the majority is fair game, but going to Alberta and mocking natives for being drunks is playing on historic and offensive stereotypes. The same is true when you tell French people to go back to France, or to stop speaking French.
And again, I go back to the canoe.ca article which essentially pointed out that saying "Go back to France" to a Quebecer is little different from saying "Go back to Africa" to an African American. Both phrases are so ladened with historic racial poignancy as to be not fair game for humour. In fact, the "humour" in using such phrases is tied not to irony, wit, double entendre or cynicism, but rather to the base distain that English Canada has and has always had for Quebec, and that America has newly developed for all things French. That's why the Toronto audience laughed, and that's why it played 'well' in the US - and that is specifically why they shouldn't have resorted to that type of mean-spirited, you-are-less-than-us-because-of-who-you-are "humour".