This is the #1 rule I always tell people who are interviewing for a new job. Never tell a potential employer your past job salary, especially so if you are unhappy with that salary.
When my girlfriend was interviewing for a new job two years ago we talked about this because the recruiter was very demanding about knowing what her current salary was and I told her to stay firm on it because the salary in her current job was, frankly, shit. In the end she got a 50% pay increase over her previous job and then six months later got promoted with a pay increase that effectively doubled her salary from her previous job. Which brings me to another point- a lot of people will justify disclosing the amount by reasoning that they can always ask for a raise after they get hired and the important thing is to get a foot in the door. The problem is that anchoring is a very real thing. If you start at $50k instead of a $75k, every raise you get at that company for the rest of your career will be based off that first salary. If you stay at a company for 10-15 years, that is an enormous difference that could be well into the six figures.
Bottom line, don't disclose salary history to employers. You'll seldom find an employer who will tell you what your colleagues in the same job make. Why do you want to show your hand?
As for this law, I'm actually mildly opposed to it. I don't think that the government should have a hand in determining salary beyond minimum wage, because that is an agreement made between two consenting parties in private industry. If you are a more experienced negotiator and are willing to ask for more money than your counterparts, why shouldn't you be at an advantage? There's no law that says you have to disclose it and the rest is up to you.