They can be, the question is entirely whether they are. I've heard it mentioned before. My understanding is there's no hard and fast rule. Some are really straight forward (the studio owns all live-action rights to a character). Some are more muddled (movie rights to a character). Some are subtle (distribution rights to a character). There's also merchandising rights. I forgot what is what, but I remember hearing that Spider-Man, Fantastic Four, and X-Men are not all exactly the same. With merchandising, I know Marvel has Spider-Man merchandising, but not X-Men or Fantastic Four. With distribution, Universal still owns Hulk and Namor distribution rights.