According to this article, it’s academic journal publishers. They describe a system that looks suspiciously like free labor and monopoly pricing:
[A]cademic publishers get their articles, their peer reviewing (vetting by other researchers) and even much of their editing for free. The material they publish was commissioned and funded not by them but by us, through government research grants and academic stipends. But to see it, we must pay again, and through the nose…
[U]niversities are locked into buying their products. Academic papers are published in only one place, and they have to be read by researchers trying to keep up with their subject. Demand is inelastic and competition non-existent, because different journals can’t publish the same material. In many cases the publishers oblige the libraries to buy a large package of journals, whether or not they want them all.
Dot-connecting to sociology will be left as an exercise for the reader.