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

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.