I'm deeply suspicious of the "tradition" either way since working at a place where it was actually a policy violation to tell a coworker what your salary was. Because then they would know if they were getting stiffed, and they might ask a raise.
The result of having a wide range of salaries in your social circles is that salary becomes taboo. There is little good to come of talking about it with non-coworker friends, somebody is just going to end up feeling bad, or jealous, or self-conscious, or asked for a personal loan... Even more closely guarded than salary is personal worth, for many of the same reasons, and more.
If this taboo doesn't exist outside the US (and I rather doubt claims that it really doesn't), I would nevertheless refuse to participate in non-anonymized discussions about salary in those settings. What good could come of it?
This taboo carries over to the workplace and into interactions between coworkers who arguably could benefit from discussing salary. If it were born just from employee handbook rules, then nobody would respect it. There are plenty of other rules in those things that nobody reads.