I assume this must be the case here because I'm familiar with a lot of different etiquette contexts in the US and I have the impression that Japan has far more of that sort of thing than we do. Off the top of my head there are (
at minimum) the way we were expected to eat in front of my grandparents, a more "regular" dinner with the extended family, a small gathering at a tex mex joint or chain restaurant or whatever, a fast food joint, and whatever slovenly things I do while sitting on my couch in private.
Anyone from a particularly wealthy family can probably add an additional couple contexts on the high end. Every single one of those situations has slightly different "rules" for what's acceptable.