Spin-polarized band calculations for LaSr7B48 show a weak ferromagnetic state. This is despite a low density of states (DOS) and a low Stoner factor. The reason for the magnetic state is found to be associated with a gain in potential energy in addition to the exchange energy, as a spin splitting is imposed. A DOS with an impuritylike La band is essential for this effect. It makes a correction to the Stoner factor and provides an explanation of the recently observed weak ferromagnetism in doped hexaborides.