"I agree with you, however, the employee was also demoted. That is what is against the law................"
Why? If it's legal to limit his political beliefs the company can also demote or reprimand him for any reason other than sex, race, or religion. He's free from being fired or demoted for having his beliefs but not from his expression of them. If I were to go around on my office trying to convert my coworkers to Christianity my employer would have the right to fire or demote me for it.
If he was going around trying to convert people, then there is a legitimate complaint. The article states he merely posted a bumper sticker, like everyone else in the office was doing. The employer can ask, or demand, that it be taken down. If the employee refuses to take it down, then he or she can be terminated. If he takes it down, and is reprimanded or demoted, and everyone else is allowed to have theirs up, that would constitute the creation of a hostile work environment. Like it or not, that is the law. The employer should not allow anything like that in the cubicles, and there would be no problem.