Not everyone works at "firms": this is extremely common in food service, for example.
> Talking about how much you get paid is not only normal it’s protected by law
Citation? I've worked at least one place where it was explicitly against the rules to share what you were paid with your coworkers (although this was years ago--I'd be happy to hear if the situation has changed).
> you should definitely be doing both legal and financial research on any perspective employers.
Obviously, but you simply don't have the same tools to do that. Have you tried asking employers for references?
The NLRB has taken action against companies that have attempted to use such handbook policies against workers: https://www.nlrb.gov/rights-we-protect/enforcement-activity/... (click the pin on Missouri, it isn't direct-linkable :()
I routinely ask my perspective boss for references. I also routinely ask for financial data before accepting a job. It has never caused even an eyebrow to be raised.