From the article: "Asked to consider his post from the eyes of a person who is gay, Klawiter responded: “I think it would be made pretty clear to me that I’m not welcome there, which I would almost consider that enlightening. I would know exactly where not to go.”"
He's pretty clear he doesn't want any gay patrons period. Posting a sign spares him the likelyhood of one sneaking through.
His business, his life.
He gets respect for standing up which is rare today in the homostapo nation we live in.