Apply this same argument to someone who spends hundreds of hours working so that they can buy a car. This is all done out of self interest. Most people would not conclude that it's okay for someone that a car thief, who steals the car because it serves his self interest. That this is somehow rational and moral because one person's self interest is equivalent to another person's self interest.

Of course people who are anti-copyright will be quick to point out that a bunch of bits on a disk that can be infinitely duplicated is different from a physical car. And that's true. But I'm not talking about that. I'm talking about the blood, sweat, and tears that the creator invests in the act of creation.

I could post more far fetched analogies. It's in a woman's interest to have self control over her body. It's in a rapists best interest to take control of her body. Does the argument that each individuals self-interest is reciprocal apply in this case? Is that the "sensible rational purely moral" conclusion? Of course not. There are a myriad of other factors that need to be taken into consideration.