I think that might be due to the free-form nature of the rules. In #lesswrong, we sometimes have lambdabot in chat, and lambdabot can evaluate (pure) Haskell functions, and also accepts private definitions of functions. So we can and do play Zendo with Haskell functions on integer triplets. Sometimes the functions are really difficult to guess, but no one seems to regard them as 'unfair'.