So what is the etiquette?

Sometimes I am very confused about etiquette and how one is supposed to respond/react. This happened in restroom today. I was attending Nature's call and next to me was a gentleman doing the same. Before we had finished our task, another gentleman (let's call him Bob) entered the restroom and said 'Hi Bill'. (Apparently, the person standing next to me was Bill.) Now first of all, I don't understand why Bob had to say Hi/Hello while Bill was concentrating on an important task. What was Bill's duty here? Should he turn back to say who is saying 'Hi'? (How embarrassing!). Couldn't Bob wait for a few minutes for Bill to finish? Interestingly, they didn't talk after Bob said 'Hi' and Bill, rather unwillingly, responded back saying 'Hm..'. So this was not a way to initiate dialog.

First of all, I don't think restroom is by any means a place to socialize. If you are seeing someone after a long while, wouldn't it be polite to wait either before or after attending Nature's call and have a small chat outside the restroom? Secondly, I think it is utterly rude to say Hi to someone from the back when you know that that person is not in a position to turn back to maintain eye contact with you.

Is it only me thinking this or there are someone thinking alike? Let me know :)