I don’t understand all the legal rules DC has to follow on this subject. But I do know that it’s PR 101 not to lie on the record when you will be easily and immediately disproven with extreme prejudice.
It may be possible that DC Comics, and all its employees, are prohibited by gag order from even saying that they “would very much LIKE to credit Bill Finger” for the partial creation of Batman or whatever, “but we CAN’T.” You know, that would be something.
It isn’t even the least they could do, if they wanted to.
They are prohibited from crediting anyone but Bob Kane for the creation of Batman - which they must do in every piece of media in which Batman appears. But they aren’t prohibited from mentioning Bill Finger. They could, if they wanted to, mention Bill Finger every single time they credit Bob Kane. Imagine if, after the “Batman Created By Bob Kane” title card in a Batman cartoon, the very next card said “Special Thanks to Bill Finger” or even simply “Bill Finger.”
More subtle than that, they could put his name, or hide it, somewhere in every movie, episode, game, and issue: On the sign of a business, in a letter, wherever. It could be Batman’s ongoing Easter Egg. Fans appreciate Bill Finger and we know the truth about the credit situation. Far from feeling like an intrusion, I bet stuff like this would be embraced by fans wholeheartedly.
But no. DC continues to be a bag of dicks about this subject, as they are about anything with which a large segment of their audience disagrees. And Larry Ganem is now on record as publicly and defiantly lying through his teeth about it. If he has a gag order, he could just say he can’t talk about it. If he doesn’t have a gag order, there’s nothing stopping him from saying what he really thinks. So maybe that’s what he did do. Even though the idea of someone agreeing with the rightness of this situation as it stands is boggling, it is possible that Larry Ganem and everyone else at DC agrees with Bob Kane that he created Batman and no one else. Otherwise why wouldn’t they just decline to answer any question about Bill Finger.
Maybe the simplest answer is that DC respects their creators as much as they do their female fans, kid fans, cartoon fans, fans of color, LGBT fans…