No one does that, because Darwin's work is science, and needs no 'preaching', nor, for that matter, is Darwin the last word on evolution.
You just have no clue.
There is a growing trend in this country that needs to be called out, and that is to label any evidence-based belief a religion. Many conservatives now say that belief in man-made climate change is a religion, and Darwinism is a religion, and, of course, atheism, the complete lack of religion, is somehow a religion too, according to the always reliable Encyclopedia Moronica. Now it's a dodge of course, straight out of the grand intellectual tradition of "I know you are, but what am I?" It's a way of saying, "Hey, we all believe in some sort of faith-based malarkey, so let's call it a push." No. No-no-no-no-no. It's not fair that people who can't defend their own nonsense get to create a fake fair-and-balanced argument, the way they do when asserting that evolution and creationism are equally valid. I'm not saying atheists are perfect thinkers; everyone has blind spots. I'm sure there are atheists who think a ponytail looks good on a man, and pineapple belongs on a pizza, and Ayn Rand was an important thinker; but when it comes to religion, we're not two side of the same coin, and you don't get to put your un-Reason up on the same shelf as my Reason. Your stuff has to go over there, on the shelf with Zeus and Thor and the Kraken.- Bill Maher, February 3, 2012