It may be threatening and annoying to those of us excited about the film, but it's not unethical or unreasonable.
It seems to me that the good people at Fox were biding their time until they could evaluate what kind of potential for return the Snyder film had. If it looked like it was going to bomb, I'm sure Fox would have been mum on its ownership of the property. Who wants to associate themselves with a terrible film? But since the film shows promise, Fox suddenly owns a golden opportunity to dip into some of that Box Office return, and make Warner Bros. look the villain at the same time.
It just looks like shallow studio politics to me.