Consider two alternative possible worlds, forking from a common worldline with equal 50% probability. In one world, an agent A develops, and in another, an agent B. Agent A can either achieve U1 A-utilons or U2 B-utilons, U2>U1 (if A chooses to get U2 B-utilons, it produces 0 A-utilons). Agent B can either achieve U1 B-utilons, or U2 A-utilons. If each of them only thinks about itself, the outcome is U1 for A and U1 for B, that is not very much. If instead each of them optimizes the other-utility, both get U2. If this causes any troubles, shift the perspective to the point before the fork, and calculate expected utility for these strategies: first one has U1/2 in both A-utility and B-utility, while the second gives U2/2 utility for both, which is better.

It's more efficient for them to produce utility for the other, which maps directly on the concept of trade. Counterfactual mugging explores exactly the same conceptual problems that you could get trying to accept the argument above. If you accept counterfactual mugging, you should accept the deal above as well. Of course, both agents must be capable of telling whether the other counterfactual agent is going to abide by the deal, which is Omega's powers in CM.