LKB3rd wrote: The law about unlawful search and seizure (4th amendment) pretty much accomplishes the same thing. Your employer can't search you or your car, and police can't enforce work rules, only laws. Correct? Unless you agree to let them search you or your car as a condition of employment.
The Fourth Amendment does not limit the right of private employers to engage in searches. Whether employers have a right to search cars on their property depends upon state law. In most states, I believe, if employers have put employees on notice that their cars are subject to search, the employer can legally do so. Also, it is always possible that someone will see a gun in your trunk when you open it, and it will come to the employer's attention in that way. While the police may not enforce an employer's private rules, the employer can fire an employee for violating the rules.
The bottom line is that these kinds of laws actually do provide some protection for employees, but they are controversial even among gun owners because they restrict, admittedly to a limited extent, a property owner's (the employer's) ability to regulate what goes on on his property. The other side of the argument is that when employers prohibit guns in cars, they are effectively preventing their employees from carrying their guns even before they get to work (assuming that there is no secure off-premises parking). So, it's a balancing of those two concerns.
In response to the OP, Florida recently passed such a law as well.