Throughout American history (and probably all history) there has always been a two-tier system of workers, one paid at the "full-pay" level, and one that is expendable in various ways.
At first, there was slavery. Slaves could be called upon to do any work that no one else wanted to do, and they were paid only subsistence.
When slavery went out of fashion, child labor came into play. And when child labor went out, women came into the marketplace. Female workers could be hired on a casual short-term basis, let go when work decreased, and they could be paid less than a man. And of course, in that same time frame, black workers were also paid less and had none of the rights and privileges of a white male worker.
When blacks and women finally gained a legal right to equal pay--not until the 1970s and 80s--there was a big gap. Who would fill in this gap between full-time, benefit-receiving, mainstream workers and the fluctuations in the market? Who would do the dirty work? That just happens to be when large numbers of illegal or undocumented workers began pouring into the country. That is also when out-sourcing began to be viable. With the internet coming of age, the niche that once would have been filled by educated but lower-paid, temporary female workers (housewives, women between graduation and marriage or child-reading) was filled by skilled or semi-skilled workers in China and India. The niche previosuly filled by "Negroes" was now filled by "Mexicans."
In academia, that very same niche, the last-hired, first-fired, non-benefit, no-commitment niche, is filled by part-time instructors.
In most cases, including that of part-time academic workers, the promise is that if you accept this low position on the totem pole, at some point, you can move into the mainstream. And then a new class of marginal workers will have to be brought in to fill your role.