There was LITERALLY nowhere around there for them to go. He just told them to leave the rest area.
In this country, the police has no backing for ordering people to evacuate. Remember Katrina? If someone is in a danger zone, but chooses not to leave, you cannot drag them out. THAT IS THE LAW. FOP will back that up too
You can keep people out, but you cannot force them to vacate unless they got their by crossing lines illegally.
Had the officer simply left, and the people been killed by a tornado, there might be some 1-800-lawyer somewhere who would pursue - but there's no legal requirement that he could go on.
(PS, what's a storm wacker? Someone that hits the hail with a baseball bat?)