There is a good write up on the WI state Bar, but it is a bit long. I just skimmed over it and have a strong feeling that by law, businesses and workplaces for reason of liability are encouraged to do less to protect. If they simply post a sign publicly for no weapons, a class b fine up to $1,000 can be imposed. But if they put in metal detectors and it fails or someone doesn’t maintain their training, the level of liability dramatically goes up. The “duty to protect” IMHO is a glorified generic term to keep people/employees safe by only protecting them from more common or more frequent occurring things – such as hazardous cleaning product spills, wet floor, icy sidewalks,……. The key terminology is “make it foreseeable”
I also think (didn’t read this yet) that there is a loop hole that gives businesses a general duty clause.
Found part of it:
WI BAR:”Plaintiffs might also try to premise liability on a failure to provide a safe workplace under 29 U.S.C. § 654(a)(1), the Occupational Safety and Health Administration’s (OSHA’s) “general duty clause,” which provides that “[e]ach employer shall furnish to each of [its] employees employment and a place of employment which are free from recognized hazards that are causing or likely to cause death or serious physical harm to its employees.” But it is unlikely that an employer’s allowance of concealed weapons, without more, would invite OSHA oversight, especially because OSHA has not promulgated a specific rule banning weapons in the workplace, and courts have held that the general duty clause does not preempt state gun laws.62 Moreover, the Wisconsin Court of Appeals has also held that the clause is “not a safety law intended to be the basis for imposing tort liability.”63”
I’ve never been asked to leave. 
