Dr. Kate correctly pointed out recently on the Thoughts From Kansas blog:
Creationists insist on calling [evolution] "Darwinism." If they can get enough people to think that science is a religion, then they can argue that their religion ought to get as much time in the science classroom as "our" religion does.
Joshua Rosenau highlights Olivia Judson's argument that "Darwinism" is a "useless phrase," and that "no sensible person ought to call evolutionary biology Darwinism (and...no sensible person does)." Descent with modification, the closely related or single ancestry of all species, and natural selection were genius discoveries about evolution that were published by Charles Darwin (photo), yes. But they were not the whole story. Much more--such as the role of genetics--would be and still is being discovered about evolution. Darwin was the first, best, greatest contributor to our understanding of evolution...but not the sum total of our understanding of evolution, and relative to numerous speculations about evolution that Darwin had, he's even been proven wrong.
(An editorial aside at this point: Technically, Judson's wrong about sensible people not using the term Darwinism insofar as in much of the English-speaking world beyond the US, especially in the UK, the term "Darwinism" is not nearly as politically charged or manipulated, and is used by many scientists and scientifically literate people to mean--and it seems more likely to understood by listeners to mean--"evolution as Darwin explained it, plus what we've learned since that he didn't know, and minus the few speculative things he got wrong.")
So why do American Creationists so doggedly insist on the "Darwinism" term to describe evolution? The real answer: political power.
Those who reject evolution as scientific fact often have political agendas relating to religion-based social conservatism. They aim to obtain power over the lives of others by attempting to make the Theory of Evolution sound like merely a cult--a Charles Darwin cult; spiteful, anti-religious adherence to some opinions Mr. Darwin penned in the 1800's. They often fear certain implications of evolution, many of them imagined--such as "social Darwinism," which is bunk and rejected by scientists far and wide--but not all of them imagined: for example, evolution's truth reveals the vacuousness of literalistic interpretations of creation myths such as those in the Book of Genesis. They fear how particular ideas of their religion-based worldview--for example, that homosexuality is a sinful choice, that political equality for women is a dangerous idea--may be complicated or even discredited by science's truths.
Of course, irrationally fearing the implications of evolution is a bit like irrationally fearing the implications of gravity. Fearing either one to the point that you'd basically deny the thing's reality is stupid; but, understanding them is empowering. Flight itself, though it might seem the opposite of gravity, is not possible without understanding gravity. Fear gravity, and you'll never invent the airplane. You might even be afraid just to wake up! ("What's holding me here in bed? Why do I strain to sit upright? Or what if I suddenly shoot upwards towards the ceiling? I think I'll just go back to sleep; it's too scary!") Similarly, fear evolution and it's like your asleep; you'll never come to understand fundamental realities about life; you'll cure far fewer diseases; save fewer endangered species; understand the complexities of our planet less; languish in a comparatively retarded sense of wonder constrained, delineated, by theology instead of thriving in an ever-expanding sense of wonder as new discoveries about life and the universe are made based on the scientific method of observation and verification.