The United States of America is the prison capital of the world, incarcerating far more people than any other nation (including China, which has over four times our population and is a repressive dictatorship). And most atheists already know that Christian proselytizing is rampant in prisons, with programs that both skirt the Constitution and blatantly trespass on it.
Given these facts, it makes sense for the atheist movement to care about what happens behind prison walls. Although atheists are known to be underrepresented in prison relative to our share of the general population, that doesn’t mean that there aren’t prisoners who are desperately seeking an alternative to omnipresent, coercive religious evangelism. Any real education we can offer them, as opposed to indoctrination with fundamentalist superstition and dogma, would be an extremely welcome breath of fresh air.