If I negotiate a contract with a child for them to work for me, it is illegal because we realize the power difference between the average child and average adult is bad enough to justify banning child labor. Yet the power difference between trained negotiators and the average job seeker is as big a gap.
Precisely, and that job seeker may be completely oblivious to the fact that the employer may be offering 100 but actually pays 200 to all workers on similar positions within the company filled by less qualified people, and may have some urgency trying to fill the job opening.
As the job seeker falls victim tho the information asymmetry, he proceeds to get screwed over and receive half of what he would otherwise get.
I do like the trend towards enabling anonymous wage transparency here.