Why is it that adjunct faculty positions pay so little? There's been a series of articles recently [1,2,3,4,5] that basically decry the dismal working conditions of adjunct faculty.

I would like to ask: Why do these positions pay so little? Is it because there is a vast amount of oversupply of teachers? Is it because the Universities have such a diverse range of topics to cover that they cannot afford to hire full-time instructors to cover these courses? Is it because there is decreasing income and funding for academic institutions?

I am curious about what conditions led to this situation where adjunct professors are paid so little.
The adjunct model seems to be predicated on an assumption that most adjunct faculty are presumed to be employed somewhere else. It's supposed to be a win-win: the institution gets a qualified expert with current, out-of-the-ivory-tower experience; the adjunct gets a chance to scratch a teaching itch, or to work with the university. All this happens for a modest compensation – which turns out to be a bargain for the university, and a little extra pocket money for the adjunct.

I didn't read all five of the articles you linked to in your question, but I did look through three of them. They seemed to be focusing on the depressing conditions for those who are trying to make a full-time living through a collection of part-time teaching assignments. I don't think that's the way the system was ever intended to operate.

Where I teach, I'm an adjunct, and I love the perks. I get to use the campus gym, and I get access to campus library resources. I have a passion for teaching, but I don't get to do much teaching at my full-time job. The extra money hasn't made me wealthy, but it's led to a few lifestyle improvements and splurges for my family. $9,000 isn't enough to live off of, but it goes a long way when you want to renovate a kitchen, take a vacation, or help pay for a wedding. 

Moreover, where I teach (a state university in the U.S.), the adjunct rates are not set by the department. The going rate is the going rate, take it or leave it. 

My brother once asked me how much my adjunct job paid per hour, if I factored in prep time and grading time. I told him that I never bothered to calculate that, but it didn't matter, because I enjoyed my duties too much to give it up. I'm fortunate in that I'm not doing this for the money, so even the relatively low pay is very much appreciated. I enjoy the challenges of teaching, the chance to experiment with new pedagogies, and the chance to make an impact on the future. 

Let me put it this way: Teaching two nights a week for fifteen weeks? $3,000. Staying up until midnight grading final exams? Zero extra dollars. Getting an email from a student from two years ago, telling you about how she's using stuff from your class at her new job? Priceless.