ejly, these are very good questions (and the mental image of all these t-shirts being worn around the world is very amusing). It seems like one of those things you can only do if you ARE that thing. It's sort of like how Rev. Lowery, at the inauguration yesterday, could make references to race that a white person wouldn't be able to get away with. We have to claim something as ours before we can start challenging and altering it. 

I am a Christian, which is a label, so I feel like I'm "allowed" to mess with that label and make sure it's one I can feel good about using. I imagine anyone who identifies with any certain group feels partly proud and partly embarrassed, because there are always going to be people who fall in the same category but have approaches and perspectives that feel very different from yours.