Why should creationism not be taught as science in public schools?

Most Christians I know don't want biblical creationism taught in science cles (they would butcher it, and then there’s the argument about all the other creation stories being taught). What we want is for microbes-to-man evolution to be taught with all its warts—in most states, they are not even allowed to present evidence that would put evolution in a poor light. And we want Intelligent Design to at least be mentioned (with teachers free to discuss it without fear of being sacked). Unlike alchemy, storks bringing babies, a flat earth, etc., a significant percentage of the (tax paying) population believes in ID (which is agnostic regarding the source of design and encompes every "creation" story).