Bill, the joke is not even about any actual real situation involving a flight of American liberals to Canada. There is no such flight, aside from a very few individual cases of some Americans deciding for various reasons to move to Canada...which has always been happening.

The primary satirical aim of the joke is to make fun of American paranoia about Hispanics pouring across the border from Mexico and Latin America, by transplanting it into a farcical, completely fictional story of "liberals" pouring across the border into Canada. Is that not funny? Is that not worthwhile satire? I think it's hilarious.

The secondary satirical aim of the joke is to make fun of the gross stereotyping of:

1. "liberals"
2. "conservatives"
and
3. Canadians

....all of whom are depicted in various utterly stereotypical ways in the joke, ways which again, have little to do with reality, but have much to do with the kneejerk prejudices of various quite ignorant and reactive people regarding "liberals", "conservatives", and Canadians.

How can you not get that? Your reaction baffles me. Were you also one of those odd people who thought that the real purpose of "All In The Family" was to glorify racism and sexism? ;-D (That was a rhetorical question, Bill, strictly for satirical purposes. Don't bother answering it, okay? I am quite sure you were not.)