I would not say they should "absolutely" not do this, but one of the things that can irritate Japanese people is talking loudly on the train. I think many tourists from other countries tend to speak louder on the trains or buses in Japan. It is an unwritten rule and good manners not to be loud when you are taking public transportation. They sometimes sit on separate seats on the train, and talk over the person between them. I am pretty sure that some people are not comfortable with it.