Is it the director's fault if the studio tells him "now your Man of Steel sequel is going to have batman. Now your movie is going to have Wonder Woman. Now your movie needs to introduce all the other Justice League heroes." Snyder is mostly to blame for length, but the studio doesn't get off scot free here.
But this isn't even fair to say that.
BvS and SS are awful because of the writing and the story telling, before anything else.
Cramming too much into one movie is a "whatever" issue, it can definitely harm your movie but if it's good content it doesn't flat out destroy it.
Cramming too much into a BAD movie on the other hand, is suicidal.