I'm not sure it affects their business all the much, and they want to show their support just like people who put signs in their yard do.
There may be a few people who would turn away from a business because they saw the wrong sign in a window, but most wouldn't care about the sign enough to go to a different store/restaraunt than they planned to. I don't think too many people would walk in because of a friendly sign either. There just aren't all that many people who have the boycott mentality when it comes to politics. How many people do you know who 'hate' Wal-Mart and still shop there?