The employer has no say within the boundaries of the vehicle. He does not own that vehicle. He has a say over that vehicle itself, nothing more. Also, such rules as OSHA exist, that encompass the rights of workers, regardless of the employer's demands.
That's how the market works...and that's how property rights work (or at least they ought to). And that is really what this issue is about.
As to OSHA, for the most part it is an overburdensome, governmental regulatory agency that sucks up untold citzens funds, and infringes on property rights. I certainly don't want more of anything like that.
Anyhow...it appears we disagree on this issue. So be it. Others can decide for themselves. But i know this, I will never asceed to the government, by force of law, and again outside of war or insurrection, telling me that armed individuals whom I would otherwise not want on my property, must be allowed there against my wishes. That is the same slope the British took with the colonists.