Because if this is so, then you are simply rejecting non-naturalistic explanations a priori. You are entitled to your opinion on this, but unfortunately, if you try to impose this naturalistic view in the education system then you are simply asserting it to be 'the truth', and so we come back to the original point.
No, wrong. We are asserting that it is the only explanation with evidence supporting it. And we are not claiming anything is truth, Truth, TRUTH, or even TRVTH. We'll leave those terms to philosophers and theologists, and others who study those squishy subjects.
As I've said before, I find nothing wrong in teaching children things in a 'this is the truth manner', and this applies equally to naturalistic explanation of, for example, the origin of life. However, in order to do this, you also have to let the children question and doubt these assertions. Which is what this' law' enables.
Will they question it on the basis of established science, or religious indoctrination? (Ever see the silly little Jack Chick tracts? That's what creationists are encouraging students to bring into science classes. What a load of nonsense!)
The fear of the creationists invasion in the schools results in the NCSE wanting to teach a one-way evolutionnary-naturalistic explanation in schools, without questions allowed or alternatives proposed.
Questions are allowed, of course. But the answers will be based on scientific evidence and theory, and that might mean that someone's pet religious belief will be excluded as not appropriate for a science class.
Alternatives? The place for alternatives, if there are any, is in the technical journals and professional meetings. Science is not
made in grade school classes. Science is
taught and hopefully
learned in those settings.
And that's where creationists give it all away: they want their religious beliefs taught as science in the school settings. (They must be getting desperate in their efforts to counteract science and all that evidence.)
And that brings us back to evidence. Creationism has none, while the theory of evolution has mountains of evidence.
Religious belief does not constitute scientific evidence, nor does it convey scientific knowledge.