The issue of Stallone and writing credit is a long and storied one.
In his book HOLLYWOOD ANIMAL Joe Eszterhas writes at length about working with Stallone on F.I.S.T. and how because Stallone had been nominated for an Oscar for writing ROCKY he insisted on having his name listed as a writer on everything since.
It's the same issue of ego as the guys who were on this show talking about CROCODILE DUNDEE IN LOS ANGELES in regard to Paul Hogan pointlessly rewriting the script so he could have his name on it.
Eszterhas also said that he didn't want to share the credit, since he had written the whole thing himself. But when Stallone found out that Eszterhas had signed a big-money deal for the paperback rights to the novelisation of the film he threatened to not allow the publisher to use his image on the cover, which, obviously, would be the thing that would sell the book. So Stallone basically blackmailed Eszterhas into giving him co-writing credit so they could use his image on the book.