It is not fair or constitutional to banish Graham for his religious beliefs any more than it would be right to fire the Muslim employees for theirs. All Christians think Islam is wrong. Do we exclude all of them or only the ones who actually express their beliefs in public? If a Muslim can think Graham is an infidel, why can't Graham think a Muslim has an evil religion. (He didn't say Muslim's were evil, btw.)
Is it possible that your view is partly formed by your own negative feelings about Christianity in general?
Graham had a right to do what he did and the employees have a right to ask their employer to not have this insulting person speak at their work site. Seems simple to me. No one has been banished. You overstate the action of not having him there.