To be fair, they are not wrong. When your main competitor is churning out one billion dollar movie after another (not to mention the tv shows, merchandising, etc etc), you can't help but go "holy crap, we have to get in on this!!!"
Marvel changed the paradigm and uncovered a new gold mine, and a lot of people were caught scrambling trying to get a piece of it. Now I agree with you at the end of the day, Justice League would never work like avengers without the initial lead up movies....we like Avengers because we organically grew with this characters. But I don't blame the DCU execs from trying to catch the comet as fast as they could, you can't leave billions of dollars on the table with no challenge.