If you don't know you're doing it wrong, why would you think you're doing something wrong?
I agree with the others that a polite conversation would be the way to go. Even a simple, "I noticed you're dragging;seem to be overloaded/probably having sway issues, have you considered trying a xxxx". Not confrontational - conversational. And don't get in a huff if they don't jump at your idea right there and then - very likely on their way home, they will notice the drag/sway/etc. and think "gee, maybe I should look into that when we get home".
And since they were from Canada and were towing in the U.S., exactly whose government should handle this situation?
