A lot of the analogies and reasoning I'm reading here is faulty. Nothing stops HR depts. from sharing information which to a first degree of approximation would give them an idea of the market rate for the positions they're filling. Companies are already free to rescind offers if they find out that you've lied about aspects of your past work history. It's part of what at-will employment is. This is about interfering with the negotiation of the individual job seeker. If I can't ask for your salary history, then I might miss out on the competitive advantage you as a candidate have in that you're willing to work for less over potentially more qualified people as you build your skill-set and expertise. The right way to help those who are taken advantage of is education about how to bargain and what information they need and need not share, not legislation.