They need to be at the advantage, their life and time is being bought. And because if the pay is shit, they'll only end up doing shitty work. Possibly leaving much not finished the next chance they get somewhere that is willing to compensate appropriately.

The same reason somebody who sells a house or a car or anything that involves negotiating should be at an advantage if they are a better negotiator. It's a fundamental part of a free market. Should the government make sure you don't sell your house for less than you could possibly get or should that be left to be decided between yourself and the potential buyer? Why should a salary negotiation be any different?