All the information we have is that there is allegedly some language in Bob Kane's contract, the one transferring his rights to
DC Comics in perpetuity, that guarantees Bob Kane and no-one else will be recognized as not only the only "de jure" ("in legal terms") creator (which is undisputed) but as the only creator
in fact.
Now, that contract is not public, and is not likely ever to be made public, as there are apparently confidentiality clauses attached. So we don't know the specifics of what
DC Comics (and Warner Brothers) can or cannot say without being in breach of contract. I find it highly unlikely, given the media upside, especially when they were working with Jerry Robinson on "The Dark Knight", that they wouldn't credit him if they felt they could, contractually.
Given this, and the fact that it has long since become public record that
DC Comics is contractually barred in some ways from giving Finger certain types of credit, I can understand perfectly well that they don't want to have this conversation again.