The argument being made is that employers have a lot more market power in the employment market than potential employees. Employers have capital, they have inertia so they can absorb a lot more risk. There are fewer employers than employees so it is easy for them to form a cartel or engage in price fixing. This especially goes for low end jobs, where a small number of large corporations make up much of the retail and fast food market. Unemployed individuals are under a lot of pressure to find employment quickly, and they know there is a lot of competition. So, employers can set wages artificially low. This power is not absolute, which explains why not every job is at minimum wage. However, this is an observable effect. The gap between rich and poor is ever widening, the middle class is eroding away, yet all the while production has been increasing. Wages are being driven down artificially low.
I wanted to ask "where does it all end?" but then I realised from the rest of your comment, you actually seem okay with slavery. Which is odd really, because you argue about human rights, but the right to freedom of movement is actually a far more fundamental and clear cut human right than the right to form contracts and do trade. In fact, if your ideals about the right to form contracts result in other people losing their right to freedom of movement, then that's a clear sign that you have lost your way.
I wanted to ask "where does it all end?" but then I realised from the rest of your comment, you actually seem okay with slavery. Which is odd really, because you argue about human rights, but the right to freedom of movement is actually a far more fundamental and clear cut human right than the right to form contracts and do trade. In fact, if your ideals about the right to form contracts result in other people losing their right to freedom of movement, then that's a clear sign that you have lost your way.