It just doesn't fit that well legally into other categories. A Jew or a Muslim can refuse to eat pork because somebody wrote it into their scriptures, and they accept it as a principle to live by. I accept it as an ethical/moral principle, but that doesn't get the same legal protection until somebody is willing to give it fair legal recognition. I acknowledge other things are fair game too, but you have to start somewhere, and it seems like a reasonable step.
I guess it's similar probably to things like conscientious objector status.
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