What are the rewards for ethical behavior by a company?
The most likely reward that a firm will get if it behaves ethically is a better public image. This will, perhaps, allow it to benefit financially as people buy more from that firm to reward it for its good behavior.
An example of this is Wal-Mart. Wal-Mart had a very bad image for years. People felt that it was a greedy company that harmed workers and communities. However, in recent years it has been improving its image with behaviors that (you can argue) are more ethical and which show a social conscience. This includes pushing its suppliers to be more "green." This has improved its public image and may be helping it remain competitive.
So, as long as firms can publicize their ethical actions, they are likely to get some rewards for such actions in the form of a better public image and more sales.