Perhaps because creationism is often taught as religious belief by zealous preacher/teachers, rather than being taught as one element commonly found in worldwide religions, and as such as something to be studied in a comparative religion class?
(That's what got the school board in trouble in Dover, isn't it?)
But then the whole point of this effort is to teach religion in the schools. None of the folks pushing creationism/creation "science"/ID in the schools is interested in scientific analyses of religion. They only want students exposed to their particular religious belief. That is what this whole sordid affair is all about, isn't it?
No, I don't think so. What this sordid affair is all about is whose religion is to be taught — to the exclusion of all others. We have already identified scientism as an ersatz religion. Atheism is a system of religious belief as well.
May I point out that creationism isn't a religion at all, it's a cosmology? And as such belongs firmly to philosophy?