Earlier quoted context omitted.
No, that's not true. They were doing something wrong, because children are generally recognized as not being fit to give consent for all sorts of things. The laws against child labor merely recognized this. The case for AB5, a law against consensual adult labour is murkier.
It is murkier, but in many cases there's a strong argument that employment is exploitative whilst still consensual.
Maybe with a labour monopsyny? I guess we don't want the big mining company in the small town to run down it's workers. Whenever I think of the best argument for unions, I think of small towns subjugated by mining companies or mills. But a ride-share company with competition in a big city?