Board Thread:General Discussion/@comment-26373999-20170331110735/@comment-24152851-20170520104611

They're allowed to make as many Easter Eggs as they like. You don't need rights from DC to give a nod to Batman or any other big name DC character. It's just having an actor actually portray the character onscreen where the rights get all messy. The furthest they ever got for easter eggs was having Harley Quinn appear in a Suicide Squad cell, but without a frontal view.

Superman appearing in Supergirl was a bit of an exception. I guess DC decided to finally be lenient on that one, since he kind of can't not appear in a show about his cousin. But it's the same reason why Superman only appeared in Season 1 as silhouettes or brief cameos.