I think the problem lies in the fact that this is a major way that people are exposed to DO's. name any famous MD (I got some examples: Bill Frist, Howard Dean, Michael Crichton, to name a few that many Americans would recognize outside a medical context).
now name any DO you've seen outside of physician visit context....they only one I can think of is Jon Marshall (hydroxycut man)
I see no fault in what they do for advertising, money talks. However, I've seen other examples of DOs being used in such marketing schemes, and they seem to exceed the number of MDs in these types of advertisements. There is legitimate concern that public perception could be swayed toward DOs being "snake oil salesmen" when they are legitimate physicians in reality.