This is an important point.^
A lot of people, especially pre-meds, look at MD and DO as basically the same thing now... if you don't get into MD school, you apply to DO. That may be the modern state where it's now sometimes used as a backdoor into any specialty, but for those genuinely interested in the actual osteopathic philosophy, I believe it is geared strongly towards primary care. It's not as if you will be shunned if you end up as a DO trying to match a specialty outside of IM/FP/Peds, but as I understand from talking to a few older attending DOs at the hospital I was training at durning 3rd year, it actually was that way not real long ago. When they went to DO school, it was assumed from the start that you were interested in, and ultimately headed for, primary care. From what I heard, their classmates who tried to match surgery, path, rad, etc were not looked upon very favorably.
...I was never really interested in DO school, haven't done a ton of reading, and could somewhat innacurate, but I do agree that a lot of students fail to realize that DO training is intended to be geared towards primary care. I have found that out more and more as I've rotated with DO students and attendings. Things seem to be gradually shifting since there are now a few dozen DO residencies for specialties outside of IM/FP/ER/Peds and many other DOs try to match with allopathic non-PC residencies. However, I think the original philosophy, not just grades, boards, politics, or degree name, is still the major reason so many DOs go into primary care...