Actually, this has annoyed me for a while now.
a GAME contains a rulebook which describes how to play. The intention of this document is to facilitate your play.
You don't have to own a copy of Diplomacy to be allowed play the board game, you don't have to pay hasbro any royalties, you just have to know the rules.
if someone introduces me to the rules and we play on a board which they have printed out, (not using a copyrighted image, but a map they drew themselves) then we're not violating copyright - at least by the interpretation of copying a book, you are allowed to read someone a book and it isn't a copyright violation.
Now patent law does protect ideas, (like the idea of how diplomacy works) and may be applicable. However while i see a copyright claim on the rulebook i see no patent claim on the idea (maybe i'm looking in the wrong place as i'm not the best patent investigator in the world)
Lastly there is trademark law, which we are almost certainyl infringing upon. The name 'diplomacy' is almost certainly protected, another company couldn't sell a product which wasn't diplomacy but call it by the same name.
Hasbro got facebook scrabble removed, and it was renamed lexico- eh, something similar which had sligthly different rules, they didn't manage to bankrupt the guys making it or stop them from competing in the face-book word games market (though i wonder how they are actually doing)
Still i'm not clear on the copyright issue. I've always thought that since a role-playing game's book is describing how to use this copyrighted material to create a story your product (the story) is yours, and you can create whatever story (film, computer game, etc.) without needing to ask permission - it is what the whole point of a role-playing system, to create stories, they couldn't exist if that was illegal.