Agree that lying is discouraged by the risk that the other party will sing, but lying - especially in a way that isn't maximally beneficial - is encouraged by the prevalence of arguments that bad lies are unlikely. The game theory of bad lies seems like it could get pretty complicated.
Update 2013-09-05.
I have since played two more AI box experiments after this one, winning both.
Update 2013-12-30:
I have lost two more AI box experiments, and won two more. Current Record is 3 Wins, 3 Losses.