Because, by the broad definition, calling a child an atheist is no more significant than those terms. When you see it like that, you shouldn't have any more of a problem with it.
I don't believe that the term "atheism" should be analogous to "apolitical". This doesn't mean that I believe that the common understanding of apolitical is incorrect.
Maybe this can clarify:
To me, the term atheism is (or should be) more analogous to "abstainer from alcohol", than "non-drinker". Abstain has connotations of choice, while "non-drinker" is merely descriptive. Both don't drink (non-drinker applies to both) but you know a bit more about the former-- s/he chose not to drink.
If you like. But what "should be" the definition of a word is not always the definition of the word, and nor is it accurate. In the case of atheism, I feel it is inaccurate with regards to the literal definition and my personal definition of the word in terms of how I define myself as an atheist. Your objection comes from your inability to dissociate your clearly personal definition of atheism, from the definition when used in broader terms. This doesn't make your definition wrong, it just makes it a personal definition which is inaccurate given the broader context that word can (and does) define.
(As an aside, it also struck me that even though babies could be described as non-smokers, etc, nobody does. It's not a normal or natural thing to do.
Yes, they do. How many babies do you see sitting with their families in the "smoking" section of restaurants (when they existed). Not to mention the examples of religious beliefs imposed on children in my earlier post. The point is that you
wouldn't object to the labelling of a child as "non-smoker", and would most likely agree that - by the definition of the term - they can be defined as belonging to that particular group. It's not about what's "normal", it's about what is true and accurate.
And yet, the argument at hand is because some people insist on applying atheism to babies. While in all cases it can be argued to be technically true-- definitional arguments aside-- it just isn't normal. So why is the exception being made with atheism?)
It isn't an exception. If you can define babies as non-smokers, then you can define them as atheists. It's as simple as that.