I don't think you understand how movies are made.
Zack and Deborah Snyder are rich as fuck. They go to WB, the distributor, and say "we'll pay you to let us make some DC movies", WB goes "OK, let's make a contract that you give us this much money up front and a percentage of the box office, and we'll distribute it for you and let you use the IP".
WB doesn't give a shit how successful it is as long as they get their money because they distribute tons of movies every year.
Then Zack/Deborah and their production company, Cruel and Unusual Films, get producers on board, hire writers, directors, actors, and crew (yes, Snyder hired HIMSELF), make a film and deliver it to WB, who then distributes it to theaters.
Every single aspect of the movie is completely under the control of the production company, which is owned by the Snyders.
WB then hopes to make a return on their investment, which they have with every movie so far. Most likely they have a longterm contract with Snyder's production company specifying a certain number of films. Most likely at the end of this contract they will negotiate a new one, in which WB will ask for more money, which Snyder will be able to provide because he's rich and these movies have made a lot of money.
The only person who can fire Zack Snyder is Zack Snyder.