Well it's not like they'd put me in jail for not saying goodbye. The problem is, everyone else does it and people kinda expect you to do the same for the sake of being polite. I don't want people to get a bad first impression of me because they considered me impolite or weird or whatever. I'm from Indonesia, and the culture here really emphasizes the need for minding your manners. Part of that is saying certain things at certain times, and telling people that you're leaving is part of that. To give you another example, people also expect you to announce it if you're going to eat near someone else by saying something along the lines of "hey, you guys mind if I eat?" and my parents have told me that eating without doing so is considered impolite. People also often gossip about other people's manners and they give high praise to people that are considered 'polite'. I don't know if these manners are considered a big deal in the US right now, so maybe it isn't that much of a problem over there. The other comments on this post seem to disagree though.