Put me in with most everyone else with fuzzy responses... the public/private line is interesting. But what's sort of coalescing for me is an intent to claim the cultural bit as your own, or the likelihood of perceived intent. So eating food is something that's done by lots of people as a way to try something new; it'd be unusual for "foreigners" to portray it as their own. But there's a strong sense of ownership with tattoos [aside from the "writing things you don't understand" issue]. With a slightly different twist, giving medical treatment puts you in a position of authority [which could be actively negated, but it would be very difficult to do so].

I don't know if that's *quite* it, but it's close to the gut feeling I have, at least.