Well we ain't lawyers so arguing is useless. Expecting honest hard work from your employees, paying them for it and f***ing with thier politics are way different things.
I really don't need to consult a lawyer to understand my rights as a property owner...and that doesn't include allowing ANYONE to put political advertising on my property that I don't approve of.
Besides if I was a lawyer, I'd probably tell you that if your parking lot is accessible to the public then it is a more public place than, say, your back yard at your house.
I talked to the cops once about people diving in my dumpster on my work property and they said it was perfectly legal. Of course, it's California.