I don't think this is a universal "in America" thing. Maybe in some parts of the country, but here in the northeast it seems like it's none of their business; I can't imagine asking someone to explain it, and I would think someone who did ask was being awfully nosy. FWIW my wife has a diamond ring which she almost never wears, and I can't remember anyone ever asking about it. My parents wear plain gold wedding bands only, and I've never heard anyone ask them about it either.