WB and DC knew Justice League sucked, but what were they gonna do?
Release it. What else. They weren't going to shelve or reshoot a film with that kind of investment.
"The studio just didn’t want to spend another dime on the film, considering all the effort that had gone into it already. They wanted to simply get it out there, get it over with, and move on to the next phase of DCU films which would act as a distinct course correction."