There's more to it than just consideration of free assn. and contracts. The Free assn element is legit, if the business states that it is gun owners they are dicriminating against.
Firearms that remain in the vehicle are not a legitimate jurisdiction for the employer, because they are otherwise hidden and can only be observed if the the disclosure is forced. Also, by unreasonable restrictions, unrelated to the workplace, that infringe on the employee's offsite activities are not allowed. This is just a parking lot remember. The firearm is only in use if it comes out of the car.
Before CCW, these rules didn't exist and folks had just as many guns in their cars before, as after. Note the catch from the recent forced searches: active hunters, target shooters and most remarkable-tobacco.
IMO, they've decided to do this to take the law into their own hands, because that's really what they have done. They are also interested in generating bogus statistics for the VPC, join together, and the other "peace" at all cost leftists, and support them.
In short, if they want to claim free assn., then let them claim that, not the bogus claim of property right.
The right to violate property rights.
"It isn't a crime f you don't get caught."