Why is it a decent number of people (Americans especially, from experience) seem to have difficulty differentiating culture from ethnicity? I've seen so many people that seem to think that if someone was black and adopted as a newborn baby by a white American family, they'd have "black culture" or something.
I think it's because up until very recently, say 50-70 years ago, ethnicity and nationality was pretty much concomitant to each other. You'd very rarely have a situation where if you did not have one you'd not also have the other. Most commonly you'd get people of half-descent from marriages over ethnic lines that then grew up with the nationality of whichever parent's nation they grew up in. These days, however, it's much less rare to see people who, for example, in America or Australia can claim to be ethnically Swedish but doesn't share a lick of Swedish culture or know a word of the language, and here in Sweden, we have people who was adopted from say China or India who aren't ethnically Swedish yet 100% nationally so.
It's also that being ethnically or culturally Swedish both share the word "being Swedish". So you got a situation where not only is these two separate concepts expressed in the exact same way but also have historically been seen as inseparate. That makes things confusing.
Of course, the timeframe above is for my swedo-centric ass. It doesn't make much sense for you Americans and colonial buttholes. But I guess also that race has pretty much dominated the scene for the last 200 or so years and put other group-concepts on the back burner.
Also that thing where Americans habitually strip all people who are even 1% visually African of all ethnicity or nationality and forces them into the "Black" pigeon-hole for all generations to come.