The case of Siegel and Shuster is slightly unique because they created the Material in Action#1 as a comic strip, tried to sell it in syndication and then repurposed the material as a comic book.
The law which gave them some leverage was designed to help creators who sell their creation to a media corporation because only the media corporation has the tools and network to sell it. (And ironically, the case of Siegel and Shsuter was one justification for the existence of the law.)
While not opposed to Kane and Kirby (and Ditko and Cockrum and etc) getting more dough, it is harder for them because in most cases they created the work as employees.
Ugh. Filled with hate for the Universe right now. Will Eisner not only deserves to be remembered for The Spirit and being one of the guys who damn near invented comics in the Forties (or at the very least the studio system)