You're completely right - it's not appropriate for a man to say "sweetie" to a woman he doesn't know. You gave the right answer.

The word "sweeite" can be used in terms of if you are in a serious relationship. Most men will their women "sweetie" or women will call a guy "sweetie" if they want something from a man or use a man for other affairs. Although "sweetie" can be offensive to both men and women also referred to as "nick name".