It is discriminatory to treat someone differently because of an attribute they have which they cannot change. So, for example, I shouldn't refuse to photograph weddings of same-sex couples, or midgets, or disabled persons etc.

I could easily see this Quran-burning pastor suing on the grounds that he was discriminated against.

How could he claim that? Federal law forbids discrimination in hiring based on religion, but that doesn't mean once I hire a Muslim worker because of his ability to perform the job I have to allow him to take out a rug, kneel and pray for an hour each day while on the clock. He still has to play by my rules, which are applied equally to everyone.