Ok I get it.Whatever. You found a way to Xploit the AI. WOW you are a supra cool...
I'd agree with this point (but not with the manner it's stated in) if not for a simple fact - it's an MP game.
In single, as long as exploit is easy to avoid I couldn't care less about it. Fighting that is about as valuable as fighting cheat software for SP games.
If it's hard to avoid (that's a hard to imagine case, but basically I'm talking about an exploit that "jumps you" while you're not actively trying to trigger it. Fortify restoration in skyrim would be a shining example of this species) it needs to be fought even in SP.
But in any game that allows OTHERS to trigger an exploit that you'd rather not trigger the witch hunt for exploits must go on regardless of how harmless they seem in SP.
On a different note, about the nature of the exploit... I think the right way to fix this would revolve around the reality of the promise.
In other words - offering a gazillion points of wealth per turn and then breaking that promise with a war shouldn't be an option.
However, offering let's say 50 per turn and then breaking THAT, SHOULD be. It's your typical real world treachery, and I see no reason for it to be excluded from the game in any way. The difference is in how believable your lie is.
That, however, should obviously be backed up by some sort of diplomatic repercussions, such as UP intervening, and the plain and old "nobody trusts you from now on" diplomatic penalty.
And it'd be perfect if you could actually choose to NOT break your promise when going to war. You know, for the sake of not loosing trust and UP not cracking down on you.