That's not how it works. You select a specific movie in a specific theater in their app, it loads the exact amount on to the moviepass mastercard, and then you purchase the ticket with the mastercard. The theater gets the full ticket price.
They claim they're going to be harvesting the data from their customers and sell that to make a profit. I personally think it's a red herring, if I were in their shoes I'd aggressively undercut the price, secure a dominant market position, and then place pressure on chains to play ball, or raise the price of the subscription in small increments ala Netflix. Honestly, it'll probably be a combination of both.
This all can sound a bit nefarious but at the same time I don't see the movie theater industry surviving without some sort of subscription model such as this. AMC, Regal, and Cinemark are squandering their market dominance by not implementing some sort of similar plan for their own specific chain (cinemark's $8.99/mo plan isn't even comprable).