I don't know, I don't think that it's polite, but they weren't very good, either. Perhaps it was an off night. Almost everyone went outside, and those that remained decided to talk to each other. Although, I have heard many people say that it's terrible that people did that, but have seen similar behavior with bands that aren't really popular, and no one seems to care.
I should probably mention this was in like 2002 that this all occurred, I don't know what was going on with said band at this time, as I do not listen to them, so perhaps they weren't really popular at that point.