Josh Rosenau over at Thoughts From Kansas explains:
The legal incentive to pair global warming with evolution in curriculum battles stems in part from a 2005 ruling by a United States District Court judge in Atlanta that the Cobb County Board of Education, which had placed stickers on certain textbooks encouraging students to view evolution as only a theory, had violated First Amendment strictures on the separation of church and state.
After that, said Joshua Rosenau, a project director for the National Center for Science Education, he began noticing that attacks on climate change science were being packaged with criticism of evolution in curriculum initiatives.
“Wherever there is a battle over evolution now,” [physicist Lawrence Krauss] said, “there is a secondary battle to diminish other hot-button issues like Big Bang and, increasingly, climate change. It is all about casting doubt on the veracity of science — to say it is just one view of the world, just another story, no better or more valid than fundamentalism.”