Is there any reason those same employees shouldn't have presumed that their contract to work for that company included what had become precedent? If they had come to work every day for the last year or more, and if they had had firearms in their vehicles before, why would they be in the wrong for presuming they couldn't continue to follow that policy?
And then there's the privacy issue. More or less related to the above, and to the presumption that we have a natural right to privacy, why should their jobs be subject to loss simply because a company decided that morning to change what had been the norm?
Because society changes as do business conditions. A private business has the right to change its business practices, does it not? Union contracts do not go on forever without changes.