CorsixTH requires Theme Hospital assets but we didn't clone or otherwise steal anything that we ship, we require you to supply the assets precisely because we aren't. I presume that's true of OpenTTD as well. In the United States copyright protection for games covers the art and text but not the rules and Oracle vs. Google established reimplentations being fair even when exposing the same api. Truely novel game rules can be protected by patents per Nintendo.