Because ID is not science, yet it is being forced into the classrooms by political means.
Some years ago, the Center for the Renewal of Science & Culture developed the The Wedge Strategy. We are now seeing that strategy in a number of different areas, including Kansas and Dover, PA. It is, in effect, bringing religion--and a single, specific religion, into public schools in violation of the US constitution.
All of this effort is not about teaching Buddhism in schools, or Old Man Coyote, or Nordic creation stories. It is about teaching the biblical version of creation. If it was, do you think it would be starting in Kansas?
Why do you think we are upset?
The theory of intelligent design (ID) holds that certain features of the universe and of living things are best explained by an intelligent cause rather than an undirected process such as natural selection. ID is thus a scientific disagreement with the core claim of evolutionary theory that the apparent design of living systems is an illusion.
http://www.intelligentdesignnetwork.org/index.htm
While your at it, can you give an example of any Founding Father banning religion in a public school?