I also disagree with your "management holds all the power". A friend of mine is a welder. He happens to be skilled enough to do pipeline and high quality construction. He doesn't work for any particular company but moves around when he feels like it and never has problems finding a job. One job he took, the immediate supervisor was a jerk. He simply walked away. Apparently, so did a lot of the other qualified welders, since the company contacted him a few months later to ask him to come back because they had fired the jerk. Since the job demands a high skill level and the supply of people capable of it does not exceed the jobs available, management had no power over them.
Skilled labor often has some control over management because of the reduced competition for jobs, not from unions. Employers often seek out skilled labor in many areas because there are a limited number of skilled workers available and they have to offer more to keep/attract them. In the case of unskilled labor at walmart, this affect doesn't exist because it is just too easy to replace someone in most places. If someone is working at walmart because it is the only job they can get, then they also need to taking opportunities to get skill training in a marketable skill. If they don't, then they have demonstrated that part-time with no benefits is all they are worth and they will be payed at that level. Direct slavery and indentured servitude do not legally exist in this country. No one, other than the individual, can stop a walmart worker from bettering themselves.