I've only worked a few times for large companies, but not discussing salary issues socially was generally considered good manners, along the lines of not showing everyone your appendix scar. You work with people who are paid more, and people who are paid less, all in wildly different jobs. What would be the point? To complain about your low pay, or brag about your high pay?

I have never seen management somehow "force" this silence on employees. What would the "implied" threats be? Has anyone ever gotten some kind of negative feedback from management because they were chatting about benefits with other employees?

Most employees have a pretty good idea of the pay scales where they work. It's not like these things are usually big secrets. There are distinct scales in public employment, and I doubt that there is anyone working in a unionized facility who doesn't know down to the penny what everyone is making. When you sign on to a larger company, you get a pretty good idea of what various positions pay. Heck, it's often on the job requirements when you're looking for work.

Maybe I'll work harder at not seeing Evil Government behind every tree if you work harder at not seeing Evil CEOs there. ;)