Supply and demand is fair, if and only if both sides are entirely free to walk away from the deal. Depending on the labor market situation this isn't necessarily a given when the alternative is starvation.
People forget, or never learned, what involuntary servitude or starvation was actually like, or what actually happened when the hammer and sickle was adopted as a state religion.
And related to the parent, in the words of Thomas Sowell, the minimum wage is always zero. Or in other words, people should be glad that the demand for cashiers still exists, until the job gets fully automated.
I find it funny, yet unsettling, that I have to state the obvious on HN of all places.
... because what happens otherwise?