There is already something odd about this question. It is like asking, â€œWhy are bachelors unmarried?â€ They are unmarried by definition. If they were married, they would not be bachelors. It is the same with ethics. To say that one should do something is another way of saying it is ethical. If it is not ethical, then one should not do it.
Perhaps when business people ask why they should be ethical, they have a different question in mind: what is the motivation for being good? Is their something in it for them?
It is perfectly all right to ask if there is a reward for being good, but this has nothing to do with whether one should be good. It makes no sense to try convince people that they should be good by pointing to the rewards that may follow. One should be good because â€œgoodâ€ is, by definition, that which one should be.
As for motivation, good behavior often brings a reward, but not every time. Think about it. If it were always in oneâ€™s interest to be good, there would be no need for ethics. We could simply act selfishly and forget about obligation. People invented ethics precisely because it does not always coincide with self interest.