Why is the capitalization of Christian and the non-capitalization of atheist a problem?

I think it bothered one person in a different question, but they might not have thought through it completely. Christianity is simply a proper name, and atheism is a term. One is supposed to be capitalized and one is not. It is that simple. Many people do capitalize "New Atheist" because of its specific reference to a movement, but "atheism" is not a religion or a movement - it is simply a term, like vegetarian or commuter. Capitalization has to do with grammar, not the importance of the word in the discussion.