Having a treatment for it around with uncertain efficacy still has strategic value. Of the subset of lunatics who might still have some, and are crazy enough to deploy it, how many would be willing to risk their attack fizzling? It shrinks an already small pool. Any case of smallpox now should be considered an act of war until proven otherwise. The only other possibilities would be a lab accident at the CDC or the one Russian repository.
Lest we automatically rule out the lab accidents we should remember the final two cases of smallpox resulted from a lab accident in Birmingham, UK. which was never fully explained. A photographer, a floor above in the building, who was allegedly never physically in the lab, contracted smallpox, presumably through air vents. She died. Her mother caught a mild case from her and survived. The head of the lab committed suicide. There'd been a larger outbreak from a London lab a few years earlier. The media, whenever discussing smallpox, rarely mention this. They prefer to focus on the "natural" final third world case, who survived, rather than the unnatural final first world cases.
BTW, smallpox is now longer the only disease man has eradicated. Victory was declared over rinderpest a few months ago, although it also survives in labs. Cattle world wide rejoice! Rinderpest, to them, was like smallpox on steroids to us - 90% mortality. Measles in humans is thought to have evolved from rinderpest