You are speaking specifically from a visual standpoint. IE; graphics. when you state look almost identical... with that logic, seven visual differences could well be in play. however, Title 17 of the U.S. Code specifically mentions graphic as well as artistic content under copyright having specific sets of rules for copyright infringement(And in fact does state specifically 10% difference, in most cases) Conversely, you are correct in stating that copying the original ROM code being illegal under current laws. However, there is no legal precedence set stating the illegality for some one to use a ROM that has been ripped. the main legality issue comes with how you procure the ROM or game, it is the illegal distribution of the games that becomes the issue. As this site is not mass producing and/or mass distributing, this would not be an issue. They are simply providing a service. Furthermore, the point I was making is that the engine behind the game is where the copyright laws would be adverted. As this would probably not be the orignial ROM (to be fair I do not know, it is an assumption.) It is extremely difficult to place copyrights on things such as game mechanics, physics engines or engines in general, game play styles, etc.
But I digress.
Cheers.