But...what if...the heroes deliberately made the film bad?!?
Before Gotham City became a hellhole, Commissioner Gordon had nothing better to do than cajole Batman and Robin to entertain the public!
So, anyway, you can believe that WB deliberately made a Superman/Batman movie that's received critical disdain.
OR you can believe that Kal-El & Bruce deliberately ruined the movie thermselves, so that DC and Zack Snyder would give back the rights, and then the heroes could go on to make the good movie which would make millions for charity. Except, of course, DC didn't realize it was a turkey, and released it anyway.
I know which theory I believe...
From World's Finest Comics #86 (1957)