The whole argument is nonsensical. The issue has nothing to do with whether the flight crosses an international border and everything to do with the service standard which DL offers and you choose to purchase for a given routing.

Rather than quibbling, it is sometimes easiest to look at the simple chart which DL publishes. It tells you exactly what you get in your chosen class of service:

http://www.delta.com/content/www/en_...omy-class.html

So, as to OP's question, if he is flying in "Main Cabin Economy" and wants to know whether liquor is free, he will see that the row "Complimentary, Beer, Wine & Cocktails" is marked "Long-haul Intl. Flights Only." The answer to his question would therefore be "no." In C+, the answer is "yes," but not because the flight is "international" but because it is C+. Pretty nifty.

The snide comments about why Canada isn't considered "international" come from simply not reading the chart.

As to why? DL could provide unlimited French champagne and top-shelf liquor in all classes of service on all flights where legal if it wished. But, it could also provide a three-course hot meal in Y and other goodies as well.