You apply for an adjunct job and you get selected to go through their "free" training. However, free isn't really free because it costs the instructor something in terms of time and energy. They move through the process and you spend 20 hours a week for a few weeks passing the training and moving into the pool.
What happens next?
No work and no training pay. You are simply put within the pool in case they have some need. They need more faculty in their pool but there never really was a pressing need in the first place.
You might say that the training costs them money. It does and there may be some small benefit of going through the training but in many cases that cost is minimal once the modules have been created. For the worker, they just did labor without pay.
What if they paid minimal wage during this training? In this case they would only put people in the pool once they have a need. The workers would receive at least some small pay, much less than they are actually worth, but there would less taking advantage of these workers.
More and more schools are relying on adjunct workers because there is no obligation beyond the initial 6 or 16 week contract and they need not pay any benefits. However, this isn't really fair to the faculty who are finding their pay dwindle and their job security shrink.