What theatres should do is offer a discount card that encourages concession sales.
Like if you buy two tickets, you get one free small popcorn. This is cheap, and because you bought two tickets, you probably brought a friend. Well now the friend will want popcorn so they'll want to buy some. And you'll want drinks because the popcorn will make both of you thirsty. Oh, how about besides that free popcorn, you get 10% off concessions to sweeten the deal?
The goal being to encourage sales of concessions where you otherwise might not have them. Someone buys two tickets, decides concessions are two expensive and that's it. Zero revenue. But hey, if we give you a free small popcorn and 10% off, suddenly you might get the sale more more popcorn and drinks. So for the cost of a few tens of cents for popcorn, you made dollars because that pair of people would want more popcorn and drinks.
If you buy bigger groups of tickets, even better - offer more free stuff. Buy 4 tickets, get a hot dog and popcorn for free, and now everyone of your friends gets 20% off. Now you'd probably buy a drink, and the friends you're with will want a popcorn, and maybe a hot food item as well.
Basically the goal is to encourage sales of concessions which make the theatre money to people who otherwise wouldn't buy them. Offering free food to a card holder is one that costs little but encourages everyone to buy more.
Even a family pack - buy a ticket for a family (3, 4, 5 people or more) and all kids get free popcorn and everyone gets 15% off concessions. Well now the parents will have to buy a drink for the kids, and if they're doing that, they may as well get something as well.