Here in Kansas, it is against state law for a business to ban guns from private vehicles, even on the company owned parking lot.
As far as I'm concerned, if it is the employer's private property, he gets to set the rules, just like I do in my home. Sure, you'll get someone who argues that it is a public place since it is open to the public, but I suspect that is either ignorance of the rights of property owners, or some type of communistic "I want my rules on his land" scenario.
I had a disagreement with one of my students one time who thought they were free to bring weapons in to the classroom. He was ignorant of both state law and the student policy he had agreed to by signing.
I, too, have never worked at a place where guns are allowed on property. It's never affected me though, as I don't carry anyway.
I'll set my rules on my land. You sent your rules on your land. Ain't freedom great?