I share your feelings about that. It incenses me to no end hear tone-deaf idiots gargling our National Anthem as though it were a throat-clearing exercise between drags on a bong. Or, some modern "artiste" engaging in up-and-down masturbatory vocalizations ending in a pitch audible only to certain species of guide dogs.
And you're also right that I never hear anyone mangle "O! Canada" in such routine fashion, even if it is a somewhat easier song to manage vocally than "The Star Spangled Banner".
Or, on the other hand, signing it as if it were a damned funeral dirge! I say, sing it as if you're Francis Scott Key, anxiously awaiting news of the battle's outcome.