Answer:
There are all sorts of examples of purposeful actions that are based on false beliefs. For example, if we see a man on his knee in a fancy restaurant in front of his dinner companion, we might say, "He is proposing to that woman because he wants to spend his life with her." One of his beliefs that motivated this action is that she will say yes. But she very well might say no. Even in that case, the man's action was still purposeful; he just didn't fulfill the purpose as he had expected. In business, entrepreneurs make faulty forecasts all the time. Their actions are still purposeful, and they still fall under the scope of economic theory.