Cashiers do not have to approach dangerous people who do not want to be approached. If they get held up they give the cash and they're good.
In cities cops are often targeted to be killed since it could be the difference between life in jail and going free.
Store clerks are also targeted because they are often alone and in poorly secure places. They have no choice weather or not to approach dangerous people because those people are approaching them. They have little or no training for handling these situations. They have no back up. They likely receive no pension or disability when injured. If they do something unwise in a dangerous situation, they will almost certainly be fired with no union to protect them.