I get what you are saying about this not being a rule, but rather an expectation that he himself set. It sounds to me that while this occurred in a poly situation you are miffed about the simple lack of common courtesy - coupled with the fact that you were prevented from dealing with it in the way you normally would.
I guess I would ask him what he would like you to do in the future when he tells you he is going to do something and doesn't follow through. Should you be able to call and confirm all is well? That he is just running late?
And again, I would ask him how he would feel if the situation was reversed.
(I am a very laid back person in general, but when someone fails to show as expected and I don't hear from them, it always crosses my mind that something may have happened to them, like a car wreck. No that has never actually been the case, but the point is you don't know.)