"...There's no need to get the police involved; the company could just fire the employee..."
Try firing a dozen men and women who have been faithfully working at the factory for decades and see if tempers don't start to flare.
In the case of the shameful Weyerhauser terminations, firearms were found locked in private vehicles during hunting season in rural Oklahoma. Most of them went hunting either before or after their shift, so it was only normal for a firearm to be inside. Several of the workers who lost their jobs said they have been leaving their guns in their vehicles for many, many hunting seasons.
This isn't an urban area where thousands of people lose jobs everyday and one just moves on to the next place. We're talking about a plant in a rural setting where Weyerhauser is the primary employer for many square miles. These are innocent parents with kids just barely making ends meet. Would YOU want to be the manager telling them to pack up their stuff and collect their last paycheck? Can you see where someone gets nervous and calls the police [government] while this is happening?
~ Blue Jays ~
I was speaking legally. Personally, I think it would probably be stupid to fire people for this, or even make the rule in the first place. I'm surprised Weyerhauser even tried it in a place where so many people own guns, given the costs to employee morale.