It's all a sad consequence of "supply and demand." Most of these jobs are tasks that can be done with little to no training (Stay at home parents do most of these tasks for free). Since little training and no certification is needed, there is a gluttonous supply of workers. When there are that many workers, there are some who are desperate enough to work for "slave wages" (less than Poverty line). That allows employers to demand incredibly low wages; because if you insist on a wage that is even semi-decent, you're fired and someone who'll work for less replaces you. It isn't fair, especially to those who are exceedingly good at these jobs, and that's the point.
The point of this work is that the economic system isn't fair in America, and to bring light to the victims. Lapham and Ehrenreich hope that readers will have the same reaction you did, and seek to change this situation, somehow.