There is another "reason" or at least justification. Corporations are not people, except as provided by law. Those laws are designed to give the people who actually own corporations protections against risking any more than they have invested. It goes back to the Dutch explorers, at least. Companies, not being people, have no rights, none. They have priveleges granted by government. In return for those privileges the government may limit what would otherwise be private rights.
And of course as others have mentioned, there are some competing rights considerations as well. In such cases the solution is often the minimum possible restrictions on the rights (or privileges) of both sides of the issue. In this case that would mean such things as requiring firearms to be locked in vehicles and not visible from outside, which is an infringement but a livable one, similarly, any objective analysis will show that no real injury at all is done to the company. (As we all know, such policies do nothing to stop criminals or the insane, and only guarantee them a safer "work environment", ie. "Gun Free Victim Zone".)
You, sir, are 100% correct. Although I've argued this point here in the past and you would be surprised at the number of people who claimed that corporations have Rights.