I think what you are discussing is school dependant. I mean let's face it, because most DO schools do not have a university affiliation, they are privately owned, and are huge money makers.
AS I understand, and this is only from what I have heard (don't believe everything I hear). At my school, if you attempt to ask our clinical education department to set up an affiliation with an institution, if that particular institution says, "Sure, we'll preceptor your student for $1000 a pop", then our school says, "No way". That is just how our school works (as I have heard at least). Why? I don't know, I don't run the place. But we do have affiliations and yes quite a few are at smaller community hospitals and or private practices, but these people are willing to preceptor for "free". Again, why our school prefers to set us up with "free affiliates", most likely to generate more profit.
So yes, as DO students we do miss out on quite a few large university medical center insitutions as official affiliates of our school but hey, that is life. My school at least does list all of their affiliates on their website, so if you were interested in applying to my school, seeing what they have to offer on their web site, there is not reason to sign up and then complain later because you do know what you're getting into. Except in one case, there was one student in my class that claimed that our schools website listed Mayo as an affiliate and then got pi$$ed later because apparently they are not. Oh well, what can you do?
Bottom line is though, these preceptors at my school openly agree to do it for free, and not all of them are DO's either. Why would they do it for free? I think they get some kind of credit for preceptoring.
Now when it comes to training residents, be aware that faculty of ACGME programs are working in clinics operated by the insitution, it is not a private practice. So if for some reason an institution starts to lose money, sometimes the first thing hospital administrators do is say, "Hey, look at how much money we are losing by running the residency's continuity clinic, if we shut down the program, we save big". In cases like that, and unfortunately I have been effected by such an action, everybody loses, the attendings are out of work and the residents need to go find a new home.