It's namely osteo schools that ask. They do it to help gauge whether you are someone who wants to get into an M.D. program and is only intending to go to D.O. school as a back-up, vs. someone who is only applying osteo. I applied to both Nova and UNECOM, among other osteo schools, and I told the truth about applying to M.D. programs (I was also asked at KCOM during the interview). I still got interviewed and accepted at a bunch of D.O. programs. It may impact the questions they ask you during the interview--if you are applying to both but can't give an explanation specific to "why osteo med?" vs simply "why medicine?" then you're probably not going to fair well.
And since you think it's none of their busines...D.O.'s who practice OMM are a dying breed, and if they just get classes full of students who really just want to be physicians rather than specifically osteopathic physicians, OMM is going to completely disappear. For the sake of patients who seek out manipulation, even though I'm going to an M.D. program, I sincerely hope this doesn't happen...then everybody seeking a manipulation will be relegated to chiropractic care, which can't offer the same medical expertise.