I wonder how frenemies live?

I am an entrepreneur and I have several business relationships. With the people that I work with, I either get along or I do not get along. There has never ever been a case of getting along while not getting along. What baffles me is when I see companies like Apple and Google doing precisely that. On the one hand they go about filing cases against each other in every court available on planet earth, while on the other hand they tend to develop applications together, buy patents as a consortium and so on.

How does somebody be at war and peace at the same time? How do you not get to a position of resolving one issue while being able to forge partnerships on another? How do you decide? It just baffles me.

I would imagine before every decision, these companies would engage in several complex calculation using game theory in order to arrive at what to do!