I was discussing this a couple of weeks ago with some co-workers. My argument was that also: someone going postal isn't going to care that the company has a no-weapons policy, and refrain from acting. I'd rather that they have a no-weapons inside building policy, or something to that aside. As it is, the state law includes parking lots as part of the company property. I think that we should park off-property so we can maintain our 2nd amendment rights, and also abide by private property laws for the company.
I have mixed feelings about the property rights issue. I do believe in the right of the owner to set the rules for people coming onto the property – but I also believe in the right to carry firearms. When I was working (after retirement – the US Army had some strict regulations about carrying non-issue firearms) it was not an issue. I worked for a couple companies that “advised” us not to carry, but I don’t recall even hearing of an outright ban.
The 2nd amendment is a non-issue, as it only restricts our federal Government, not individuals.