Never claimed you did.
How did evolution get associated with religion in the US?
Fundamentalists have had a problem with it ever since Darwin. Any science that challenges their (narrow, overly literal) understanding of the scriptures is relentlessly attacked, in the political arena. These guys want
everything - the family, the school, the church, the state - under their divine control.
Probably the big turning point was in the 1920s in Tennessee, when the state passed the Butler Act, a law prohibiting any state-funded educational institution from teaching "any theory that denies the Story of the Divine Creation of man as taught in the Bible." This culminated in the Scopes Trial, which sufficiently embarrassed Tennessee to get them to repeal the law (although a subsequent Supreme Court case affirmed that such laws are indeed contraventions of the Establishment Clause).
After that it died down somewhat for a while, with even the Pope coming around to the side of science (sorta), until the current intelligent design movement got up and running. This -
again - represents a systematic, politicized attempt by religious fundamentalists to interfere with science education and policy, at the local, state and national levels. For example, Kansas's removal of evolution and the Big Bang from science cirricula, Georgia's attempt to remove the word "evolution" from the science cirricula, etc. There are church-funded "student clubs" at every major (and most minor) universities in America dedicated to legitimizing intelligent design and other creationist hooplah as legitimate subject material, along with national marketting and outreach organizations to back them (all of it funded by religious fundamentalists).
In my own short sojourn in the states, I have seen the issue hammered in more by atheists than theists.
In my lengthy life in the States, I have never seen the issue "hammered" by anyone other than evangelists.
I do know plenty of scientists and educators (of all faiths) who are sick of having their occupations hammered by ignorant fundamentalists, though.
The entire movement of associating an acceptance of evolution with atheism seems to be pro-active in prominent atheists
There is no such movement - generally the message from the non-fundie side is about how evolution is totally compatible with non-kooky understandings of religion. The only people I've ever heard equate evolution with atheism are the religious fundamentalists trying to outlaw teaching of evolution (and the Big Bang, etc.). What you are doing here is endorsing the framing of the religious fundamentalists: that all those who oppose them are godless heathens. This is the crucial lie, on their end, since most religious people in the US do not share their agenda, and so must be marginalized in order to empower the fundie campaign to speak for "religion."
And there aren't any "prominent atheists" in the US. We have no Dawkins, for example.
Not that the concept of a "movement" being "pro-active in" a person makes the slightest sense in the first place... presumably you meant to refer to people pro-active in the movement? Again, however, there is no such "movement." The organized campaign is entirely on the fundie side.
By pointing out that atheists conflating atheism with science is detrimental to science?
No, by evangelizing absurd fundamentalist suppositions like that one. You have yet to produce an example of an atheist conflating atheism with science (or anything else). All you've got so far is one theist (yourself) misconstruing science and insisting that this is somehow a critique of atheism.