What exactly are you referring to when you compare supplies costs between supermarkets and cinema?
How much more does pop-corn cost to the cinema to hit its mark-ups? Seriously, it's corn, it costs next to nothing both for the cinema and the shop/supermarket. It's not as if the margins are squeezed for the cinema here compared to the shop.
Also, all fizzy drinks are basically drink fountains based on syrup+water, again I would be quick to suggest that it might be cheaper for the cinema to procure these (they come in suryp packages) rather than the bottles the shop buys.
What are we left with then? doritos and hot dogs, ok they may be paying more there - or not?
Really, the food/drinks is the profit making machine for the cinema, that is understandable, but there is no argument that it costs them more than other retailers. The problem is that ticket prices are not profitable enough and they have to milk the sales of foodstuff to make up for it.