So I have a little bit of a different thought about this. I don't think that you're specifically saying "smile because it makes you look prettier" or anything. But I also agree that it can definitely come off that way.

I think it's more of a miscommunication. A man sees a woman who looks sad, and wants to make her feel better, so thinking by telling her they're prettier if they smile will do that. Unfortunately, it won't, because he's obviously ignoring a lot of factors (resting bitch face, maybe something is wrong in their personal lives that they don't want to talk about, maybe they are just eating fucking lunch and don't want to talk to a rando, and most of all that you don't think that they don't look pretty when they look like they do which isn't any of your business).

I think it's usually more that a man finds a woman pretty and happens to look unhappy, and the misguided gesture is to try to make them feel better by telling them that he thinks they're pretty by saying that they'd be prettier if they weren't unhappy, without regarding any other factors. I guess it's all about intent, as are most things. It's probably better just to not say anything about a strangers looks at all, male or female.

Not totally related story, I once told Carrot Top he looked awful when I waited on him at a brunch place in like 2002, turned out he had a bad head cold. I cringe now when I think about it though, I could have easily offended a stranger much worse if it were something else.