Stepping on a land mine

Yes, that's what this post will be. But I really don't understand this and maybe someone can help me out.

Why is it okay for Oprah Winfrey and Bill Clinton to go to a church and stump for their favorite politicians and it is not okay that someone like Mitt Romney (not an endorsement) can't talk about the fact that he is running for president at church?

I understand (like some do not) the separation of church and state. That part of the constitution was written to protect churches from the state, not the other way around, by the way.

What I don't understand is why is it allowed for some and not others? I don't get it. Maybe someone else does. Answers? I am not looking for a political discussion. I'm really just looking for a good answer.