So why do I have a problem with atheists saying they're spiritual?
Even if there were not this huge and growing movement of people who insist that they are "spiritual but not religious," "spiritual" would be a nebulous term, and, it seems to me, clarity of thought and expression is one of the primary goals of the atheist. Or at least it seems to me it should be. To say the same thing another way: clarity and religion are mutually-opposed principles. One lessens the other. What does a person mean when he describes himself as spiritual? Who knows? Perhaps an atheist describes himself as spiritual, and by that he means that he feels a great sense of awe when contemplating the universe. (And by the way, who doesn't?) If that's what he means, then that's what he should say. Eleven words: "I feel a great sense of awe when contemplating the universe." Instead of five: "I am a spiritual person." Six extra words are a small price to pay to eliminate so much confusion.
And of course the above-mentioned "spiritual but not religious" people, who of course are nothing more or less than religious people whose relationship with some religious organization has undergone a change, and who are not aware, or behave as if they were unaware, that what they are now is already covered by some existing category of person traditionally considered just as religious as anyone else, if not actually more so: Protestant, or mystical, or Pietistic, or charismatic, or some combination of the above -- these people provide one more big reason for atheists not to describe themselves as spiritual.
Of course it makes no sense for these people to say that they are not religious -- that is to say: not religious anymore. Have you ever met any "spiritual but not religious" people who were not previously religious, according even to themselves? Yeah, neither have I -- but, of course, language does not change according to what makes sense, but according to how people use it, and I am already resigning myself to these turnips re-defining a few terms for all of us. I'm fighting them on linguistic and logical grounds, but I'm not kidding myself that I'm going to win.