I fly alone with three kids in and out of Germany a lot, and have never been asked for a letter. I did get scolded for arriving in Germany without my French passport but that was another story (would have gotten away with it if I hadn't been flying with three French children lol!)
I actually never needed a letter for any country but I've never gone to Canada with them (over it, but not too).
Most countries don't need them and those that do, are very specific about what they want. I think you got lucky. I wouldn't chance it with Canada. I bet Canada is sick of dealing with American abduction cases so that's why they're checking arriving solo parents.
Interestingly, I don't need it for France but they once ran a check on me. I didn't even realize it. I rarely fly out of CDG and they announced "You are allowed out of the territory". Ahem? "...With the children. You're not on the list..." oh thanks?
No letter will stop a parent from simply not coming home from a visit to the home country.
I just want fellow solo traveling parents to check the specific country where they are headed and not just write up any old general letter thinking that they have it covered, only to have it not accepted.