Maybe because to entertain the idea of discussing anything other than "common descent" makes common descent suspect.
They are supreme in all matters of science with their ideology and the stupid ignorant ID'ers and or creationists are not going to be allowed to share in the money pie that is public education K-12.
Their prophetic warnings of doom and destitution to third world status exposes the core of their belief. Talk about mind control.
Maybe because to entertain the idea of discussing anything other than "common descent" makes common descent suspect. Your premise is flawed, in that it says one must allow an unsupported argument to automatically receive undeserved credence because if someone challenges its validity, it wins legitimacy simply by being refuted.
Using this logic, if someone claimed the moon might be made of cheese so it should be taught as an alternate theory, and someone else opposed teaching this on the basis that it is a ridiculous claim without merit, then would the very act of opposing the Cheese Moon Theorytm make the notion that the moon is made of rock 'suspect'?