They don't want thugs, that's all. But they can't come right out and say, "no thugs" so they have to implement this convoluted dress code in an effort to discourage them.
And they want the type of customers who also don't want thugs. This is Chicago, after all.
That may be their objective but they spread too wide a net with that description. A phrase like “proper dinner attire expected” followed by “management reserves the right to determine what is and is not proper dinner attire.” is all that’s needed. Doing this just gets all the Jesse Jacksons to come out.