I don't think most are. I know my program certainly isn't. At the one place I went to that had a distinct "MD only" interview, I was still being evaluated as an MD/PhD applicant. The difference was just that I didn't spend the bulk of the interview discussing my research.
I, like plumazul, checked off (or didn't check off, don't remember) the box indicating that I DID NOT wish to be considered for MD only at every school but still had a couple offer MD interviews or asked me again after I got rejected from MD/PhD if I wanted to be considered for MD only. I personally didn't accept any of them because my plan B was to reapply (my employer would have happily let me work for them another year or two - don't know if the reapply option would have been so appealing had I not had that job as a fall back), not make due with MD only but to each his own and there's nothing wrong with pursuing the MD only route as a backup.