The one thing that all of these teachers have in common is that they are less expensive to use than tenure track professors. Also, I think it is generally true that they regard being able to say they are "professors" is a big deal to them.
I am not saying this is exploitation since these folks have choices but there a few things that seem amiss. (I do think the adjunct route is better for schools in highly populated areas were the choices are better, being able to say you are a "professor" is less important and the main qualification is not knowing someone on the faculty. )
First, what kind of rational hiring process spends tens of thousands of dollars in search expenses for professors on the one hand and conducts no search for those who will teach even more. I am not saying one is better but it's not a case in which the mix makes everything better.
Second, if the idea of a search is to ensure diversity and fair opportunities, why, if you take one position that involves teaching 3 courses and divide it in thirds, does the need for or desirability for a search disappear?
Third, part time teachers are cheap and seem desperate for the opportunity. Many have no say in governance and little contact with the school other than fitting in after work. Does this mean that power gravitates to the administration. More importantly, is that really a bad thing?
Distance learning, on line courses and degrees, externships, and part time teachers all involve outsourcing of a sort. The problem (or rather, the explanation) is not that it is driven by money grubbing management that hopes to make shareholders happy by cost cutting. In this case of outsourcing, no one gets richer.
I am not sure where this goes or even if I think it is wrong. I do not like it but that is a different matter. Most of this is cross-posted on classbias.