The only force acting on the penny when it is dropped is due to the mass of that penny. Weight is the only force acting on penny. In free body diagram of penny, you will show only those forces which acts on it. You will not include unnecessary forces such as intermolecular forces between the atoms which constitutes any object or in this case penny because we are not concerned about it. The penny falls down because an unbalanced force is acting on it which is due to gravitational attraction by Earth, since the penny is falling vertically downward and not in any other direction, the net force acting on it should point in downward direction.
