I get why employers would hate this, but overall, wouldn't greater transparency in the labor market drive economic growth by virtue of making the labor market more efficient by doing a better job of connecting skilled, hard-working and motivated employees with employers who can pay more?
I don't think they'll hate it. If they know what a prospective employee is making, they can offer a minimal increment on that within their (wide) salary range. Currently, only the employer knows their real budget (and that may change if they get the right candidate) and only the employee knows what they'll move for. Salary negotiation is based on those two secrets. This legislation makes the latter secret public know…
Not everyone's motives is the same on this, so it's still secret. Some people will require more than a small bump to move into the risk of an unknown new employer.
Also, some people value flexibility (or rigidity), autonomy, etc. more than money - to the point where some people will pass substantial amounts of money if it means sacrificing real time with the fam.