you communicate your bridge hand through hand signals, you're a scummy cheat - even if it helps you win. I don't do that, because it's wrong. Certainly, making private real-world side deals strikes me as cheating

Bridge specifies that communicating information about your hand is against the rules; Diplomacy says that making deals is specifically part of the rules. Diplomacy doesn't provide an enforceable contract, sure, but that just means that finding a method of creating enforceable contracts gives you an advantage.