I'm an adjunct, and I've written about various aspects of being an adjunct (e.g. here: http://jakeseliger.com/2014/12/22/how-do-you-know-when-youre...). But I'll add that there is a very good "excuse" why universities treat adjuncts how they do: because they can. When people stop signing up for grad school and/or to be adjuncts, universities will have to offer better pay and/or conditions.
Until that happens, universities won't. Markets are clearing.
Until that happens, universities won't. Markets are clearing.