Cooperation is a non-zero-sum game that continues adding utility the longer it goes on. Do you deny that this is the case?

That is sometimes referred to as "mutually beneficial" cooperation:

Co-operation or co-operative behaviours are terms used to describe behaviours by organisms which are beneficial to other organisms, and are selected for on that basis. Under this definition, altruism is a form of co-operation in which there is no direct benefit to the actor (the organism carrying out the behaviour). Co-operative behaviour in which there is a direct benefit to the actor as well as the recipient can be termed "mutually beneficial".

• http://en.wikipedia.org/wiki/Co-operation_(evolution)