I fully understand where it leads. However at some point, there is a reasonable limit that the employer can do.
It wasn't reached in this case, and I don't suggest it was.
If the employer has trained them properly, ensured a full understanding of the policy, and done everything they could there should be a reasonable expectation of the employees adherence to what they have learned. Once they pass outside that particular scope of behavior they should be solely liable, not the employer who made it clear that such action was outside any acceptible action.
You see to me, if you are conducting business that requires work be done that is inherantly dangerous... Driving forklifts where people are shoping. Then you should be responsable for the injury that results. You are benefiting from the business, you should share in the damage it causes.
If you have employees who are running forklifts you should try to make it as safe as possable... the only way to ensure that a company will try to make it as safe as possable is to make them responsable for injury that results.
I agree with the law, that if an employee goes off and does something not in your interests, like goofing around and throws a cherry bomb down the toilet, you are not responsable.