Since I don't live there, is Illinois a right to work state? Also you do have to be some kinda stupid to wear Packers/Wings shit to work in Chicago.
Illinois is a fire at will state, unless some other binding contract sets firing rules.
I find the firing unusual, unless the person had no other additional employee issues of insubordination. It also depends on what Webb's dress code is. If dress code has wording along the lines of "at the discretion of the manager" or "no logo's other than Webb", then the manager has the right to ask him to take the tie off (failure to do so after prompted several times is insubordination). A reasonable manager would not, but one would be surprised...
But honestly if a customer is so put off at someone wearing a Packers tie has other serious issues. Hell it is kind of a talking point, really, and favors the salesman as a redirect, etc.