Not fear for themselves — they are too strong in their faith to be corrupted by evolutionary science. It is fear for their children and in particular, fear for their children’s souls. There is a genuine belief that accepting an evolutionary view of biological phenomena is a giant step on the road to atheism, and in learning evolutionary theory their children are in peril of losing salvation. Given the beliefs they hold, this is not a silly fear. From their perspective, atheism is a deadly threat, and evolution is a door through which that threat can enter to corrupt one’s child. No amount of scientific research, no citations of scientific studies, no detailed criticism of the Wellsian trash science offered in “teach the controversy” proposals, speaks to those fears. If one genuinely fears that learning evolution will corrupt one’s children and damn them for eternity, scientific reasoning is wholly irrelevant.
I can attest to this from personal experience, not only from when I came out of the theological closet and announced agnosticism to nervous parents, but long before that.
When I was in high school, a creationist group approached the administration with the plan to hold lectures on Intelligent Design. I attended the heated school board meeting, in which, despite threats (real or imagined) of an ACLU lawsuit, it was decided that the presentations could be held on school property, but not during school hours. At the time, I was completely oblivious to the larger political, social, or educational issues--but it was clear, to my then-Christian mind, that this was a triumph.
So much for my personal, historical perspective.
I disagree that scientific reasoning is "wholly irrelevant"--otherwise, creationists* wouldn't labor so hard to obfuscate, misrepresent, and distort the facts, constantly appealing to science when it, out of context, is in agreement with their position. People look up to scientists and respect scientific pronouncements.
I agree that fear is a strong motivating factor. But there's another angle.
Simplified creationist presentations, in their gee-whiz fashion, constantly equate ignorance and wonder. They'll say, "We don't know how the woodpecker could possibly evolve--isn't it amazing? Your body is an assortment of trillions of irreducibly complex machines--aren't you special?"
Wonder, as I'm sure even most ID-touting theorists would agree, isn't the exclusive province of the baffled.
Ultimately, we are here. No matter how we got here, that's reason enough to be amazed.
*I might be accused of conflating different strands of creationism--but I'm referring specificially to those who use ID as a prop for Christian apologetics.