No one is saying that an individual can't carry a gun in their vehicle...they just can't park it on another individual's private property because the property owner's right to set the conditions of access and use of his property begin at the property line.
Your entire argument has always been that someone other than the rightful owner of property can set rules and conditions on land that belongs to someone else.
Yours is not only a ridiculous argument, it's also circular, because it relies on the same property rights that you are arguing you should be able to violate.
What about the basic Rights of the vehicle owner?
And don't give me this "he can go work elsewhere" crap.
Under such logic, a kompany could do the following:
Fire a woman for refusing to sleep with the boss.
Fire an employee with 40 years service on the day before he retires so that the korporation won't have to pay out a big pension.
Require that all employees vote for a certain candidate for political office.
And so on. And kompanies have done ALL of these things, many times over, which is one reason they are regulated. When dishonest korporations screw honest folks over, gov't gets involved, as they should when the Rights of individual citizens are at stake.
Even if the employee signed a document, before being hired, no lawful court would recognize it, since no sane man or woman would sign away their rights, unless due to fraud or force. You can't sign away your basic rights. Any such contract is "null and void".