Yes, it's work.
But the lawyers are correct.
IF the employer allows SOME people to post political messages in their cubicle, the employer loses the power to prevent OTHER people from posting political messages in theirs, so long as the messages are not obscene.
Employers have the right to say "NO politics or displays of religion at work." That is perfectly legal and legitimate. But they do not have the right, if they allow some of it, to ban others of it. Once the door is opened, it's opened. The employee will win his case, because the employer does not have a leg to stand on here.
employers should be able to fire or hire based on whatever reason they want. It's their business.
Unless the bumper sticker is over the top with regards to obsenity or it is extremely offensive. That is why I asked what does the bumper sticker say? My guess is that this is not the case and you are probably right...
The employer is completely within his rights here. He can allow or disallow ANY message he wants. There is no "fairness doctrine" in the workplace. A private business can't fire him for his religous views but it can limit his expression of those views in the workplace, even if they allow others. The only equality laws that apply to the workplace are hiring and compensation related based sex, race, or religion. That's all the law covers. Thank God. It is the employer's company, not the government's. His actions may have negative consequences but that's his problem, not Uncle Sam's.
So a Christian bookstore, for example, that lets employees wear buttons with various Christian messages, must allow employees to wear buttons that say, "Up with atheism", or "Nietsche was right"? You're probably right that a court would enforce exactly that, but it's a sad, sad loss of freedom in our country. If I want to start a company and, being my private property, hire only people that agree with my religious and political views, that should be my right.