I've seen many 3D movies in theaters, and they never charged extra for the glasses. Where are they doing this? It would only make sense if there was an option to bring your own, but that's not the case.
They charged me extra for Dune at least. 20 euro ticket + 2 euro for the glasses which you keep in Berlin. Last time before that I had been in Bulgaria a few years ago and there it was just part of the original ticket price. So I guess it matters per location but I think the profit margins on the glasses are tiny compared to on anything else in the Cinema anyway.
I don't know. After asking people around and checking online plenty of people seem to prefer 3D and allegedly it sells more seats when there are 3d viewings.
Wrong incentives at play. It's not that people want it, they force 3D movies on you and charge you for the glasses. Easy money.