This is great law.
If they allow for the possibility that the employee can defend themselves, they are not liable.
If they prevent the employee from defending themselves, they are liable.
And that's the way it should be. An employer's private property rights do not negate the right to life and self-defence that the employees have. Simply because someone agrees to work for you doesn't mean you have the right to put their lives in jeopardy if some nutjob comes on site and starts shooting people down.