As an owner of a company, Nobody has the right to tell the owner, what kind of legal weaponry can be brought on the owner's company property. If the owner wants bazookas on the property or no guns at all, the property owner gets to decide. Case closed.
It sounds like you want to give property owners the right to search personal vehicles and decide what items people can and cannot carry in their vehicle. That is a path heading down a slippery and dangerous slope. When you run a business that employs people, you unavoidably take on certain risks associated with your employees, including the risks of actions those people may take on company property. There's no way to avoid those risks. If an individual or a company doesn't want to take those risks, then they have to hire contractors and consultants to do work for them on the contractor's property.
I'll tell you what these restrictions on guns on company property are really all about: reducing the threat of litigation by trial lawyers. These restrictions on guns don't actually make employees safer, but if an employee is shot on company property, the restrictions make it easier to defend the company against a lawsuit filed by the family of that employee. The company can then argue that they had restrictions in place against guns in cars and they "did all they could to prevent this tragedy." While the restrictions are of course ineffective and do not stop criminals from keeping guns in their cars, this sounds good in court and it can help to defend against lawsuits. That's all this is my friend. It's corporate risk management at the expense of employee rights. It's nothing more and nothing less. Have a great weekend.