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.
If you believe what MoviePass is saying, they’re not losing money hand over fist. They claim most people stop going to the movies after the first couple of months. If someone doesn’t see 12 movies in a year, they aren’t losing money. They say the average is closer to 4.
When the subscription price was higher, they have fewer members and the people that signed up went to a lot of movies and MoviePass was losing a lot of money. When the dropped the price, a lot more people signed up and most of these were casual movie goers.
In every way they are better off with two million subscribers that go to 8 movies a year than a hundred and fifty thousand subscribers that go to 25 movies a year.
And this is exactly why they pulled out of the top AMC theaters. No surprise they're also the most expensive ones. The AMC at citywalk in LA is almost 20 a ticket for a basic 2d showing.
It's apparently not the ticket price that they care about. They removed support for the River East 21 in Chicago, which has 21 screens and shows a lot of foreign and limited releases. You could probably go there every couple of days and see a new movie you haven't seen before. Ticket prices there are actually cheaper than the 600 N. Michigan AMC which they still support. But that theater only has 9 screens and basically only shows blockbusters.
Likewise, the Webster Regal in Chicago is about $3 more per ticket than theater they dropped support for. But they don't carry all the limited/foreign releases that River East does.
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.