Due to the way the legal system in the US works, there is a lot of posturing. Plenty of the forms you sign are not enforceable at all and only there to make people ignorant of the relevant laws afraid to exercise rights they actually have.
>Plenty of "serious" firms ask about last salary earned (in fact, if you were to fill out an application on line
Are you saying that a "serious" firm would have you fill out an application (online or otherwise)? I thought any career level job would expect a resume/CV.
Big finance firms (EG, top 3 in the world) require tax documentation and will rescind offers if your comp was exaggerated.