The subject here is etiquette, not morality or principle. That doesn’t mean morality and principles aren’t important — they are. In some cases, they may be more important than etiquette. If that’s the case, you should probably do the right thing instead of the polite thing.
Standing up for rights and freedoms is often impolite, but often a moral imperative. Gandhi’s and MLK’s disobedience was civil, but it wasn’t polite. Throwing tea into Boston Harbor wasn’t polite, and ridiculing the latter-day wannabes as “teabaggers” isn’t polite (but it sure is fun!).
Even so, when you’re doing something impolite — even if you’re completely in the right, even if not doing it would be immoral — it’s good to be aware that you’re being impolite. Knowing something is better than not knowing it, and knowing you’re being impolite can help you try to deal with the fallout from your action (or non-action).
But lots of times, it’s not a moral issue at all, merely an issue of getting along with people, and maintaining good relations with them. Just as religious people have to get along with people who they believe are going to hell. That doesn’t mean they have a moral imperative to bash them over the head with it every chance they get.
And for atheists, just as for gays in times past (and unfortunately, in many cases, times still present), being “out” can carry heavy consequences. People may disagree about whether being “in the closet” is always an act of cowardice or sometimes a sensible way to get along in life without setting oneself up for unnecessary headaches and problems. But hopefully we can all agree that people who choose to keep their atheism under wraps may well be going through something that we have no right to judge if we haven’t walked a mile in their shoes.
(pic of nasty letter via Not A Potted Plant)