The marginal cost to a theater, sure. But, as the 3rd paragraph points out, MoviePass is paying those theaters full price for each ticket. They're losing money hand over fist.

They can't just solve this by squeezing theaters, either. They're already struggling, if not barely surviving. If MoviePass simply tries to strong-arm them into accepting less than full price, without offering some other source of revenue in return, that's a lose-lose strategy for them. If they fail, they lose their ability to supply customers in that market with the service that they signed up for. If they succeed, they risk bankrupting the people who actually supply their customers in that market with the service that they signed up for, and so they still lose it.