The opponent may be Genghis and maybe some people find this fun, but come on, we're playing a computer here. I'm more inclined to believe that this is a bug, as it happens very often with a lot of other civilizations as well.
I had Elizabeth, Napoleon, Caesar, Ghandi, Isabella, Boudica, Alexander and Dido all lie to me about spying and some of them were my friends (DoF) at the time. What is even the point of them promising not to do it again? This AI behaviour has now has lead me to believe that all AIs lie all the time and therefore I should just DoW and conquer them (if possible) as the only means to stopping their espionage (until the Great Firewall anyway - but who plays that long?). I don't find that one bit enjoyable or fun.