Because how many bakers or photographers refuse to bake cakes or take pics of the weddings of adulterers? Or fornicators? Because almost everyone sleeps with their fiances before the wedding, even lives with them these days. And I'm sure there are many cases, esp. in small towns, where these service providers are aware of cheaters.
Very very few who believe that these are very serious sins, on par (according to the Bible) with homosexuality are refusing service to the other sinners. But they are CHOOSING to refuse service only to the gay "sinners."