People don't talk about salary in the US because companies will fire you for it.
Edit: See kasey_junk's response and my follow-up.
"Your right to discuss your salary information with your coworkers is protected by the federal government. The National Labor Relations Act states that employers can't ban the discussion of salary and working conditions among employees."
http://work.chron.com/can-tell-coworkers-salary-7204.html
Edit: Just saw that response and while I agree that you can be fired for any reason...that could be for anything at all. The only reason most companies are going to fire you is if:
a) You aren't doing the job you were hired to do b) You are creating a negative work environment. Actively trying to discuss salaries publicly at the company could definitely do that, depending on the type of and size of the company. Creating resentment definitely can cause problems but it's all situational. The situation that you saw first hand could have been as simple as you describe it or there could have been other contributing factors that you aren't aware of.
Being fired on a hair trigger for something like that seems extremely unlikely. Businesses don't go through the hiring, interview, background check and training only to fire somebody because they went against a rule in the book. Companies hire people for to do jobs because they need them to do that job and they value that performance.
There's probably some lack of teeth issues, but I think the biggest issue is that people are just culturally conditioned not to take advantage of the protections that exist.
I've also seen manager training materials that specifically tell managers not to even hint that salary is a secret topic as the liability involved is problematic.
It is one thing to claim that worker protections don't exist, its another to claim that you refuse to take advantage of them.
I've seen people get fired for being late to a meeting once. The real reason they got fired was something else but on paper and what they told the person was for being late.
http://www.npr.org/2014/04/13/301989789/pay-secrecy-policies...
"Under the National Labor Relations Act, enacted in 1935, private-sector employees have the right to engage in "concerted activities for the purpose of collective bargaining or other mutual aid or protection."
The language is somewhat antiquated, but according to Estlund, "it means that you and your co-workers get to talk together about things that matter to you at work.""