It's right in the sense that it's true, but it doesn't answer the question. Just because the theater gets most of its money through concessions doesn't automatically mean that concession pricing is exempt from competitive pressure or supply and demand. Of course theaters want to charge as much as they can for their product, as any profit-maximizing firm would, but the question (a good one) is why the market allows it.
That's odd, because the economists are agreeing with you - in your analysis, that is, not in your opinion of economists. But further than that; the microeconomic theories are more generally applicable.
(Me? I hate the stink of popcorn. I'd pay extra to go to a cinema that didn't sell it.)
Amazing the answers you get when you throw a bunch of economist at the question who know nothing about the industry.