I think that bosses aren't concerned that if they confront a worker for his poor performance, substance abuse, etc. and fire him that he won't go out to his car, get his Bible, and take it back into the office to use against his coworkers.
Parking lots are private property belonging to companies or real estate managers. If they want to ban weapons from those locations, they have the right to do so. Gun-owners who want to keep guns in their cars can quit and find companies that allow it or look the other way. That's the American way--not this state-enforced tolerance.
As opposed to going home and getting a firearm and going back into the office etc? Your argument is specious.
Your statement of the employers concern is obvious. My question regarding the bill of rights is, so far, unaddressed by your response. Freedom isn't comfortable. If one has the freedom to keep a bible locked in the car under the guaranees of the first amendment, how does one lose ones rights under the second.