Why is it not necessary to include a negative control for this stain procedure?

It is not necessary because endospores stain is a differential type of staining procedure that permits to perceive together spores and vegetative cells. Therefore, comprising a separate negative control containing of only vegetative cells is not essential. In addition, the use of an endospore stain is to see specific cell organizations that can tell if some bacterium cells comprise higher resilient spores in vegetative cells.