In the United States, this is not just unnecessary secrecy: this is completely illegal. Trying to forbid employee-to-employee conversations about compensation will land the company in trouble, and every manager should know this.
I understand this to be a fairly common experience in the U.S. workforce.
When it comes to labor laws in particular, the rate of enforcement is extremely low.
You agreed to the pay. Want more? Ask more. If you're making 2x more than your "peers" would you take a cut to equal pay?
Information is missing on both sides.
Contracts are open with all Information available. To that route if you're wanting to make more than the W2
Knowing salary is also useful for existing employees, and in that case (in the US) the employer likely does know all of those things.
If I’m paid twice that of my colleagues, I’d want to support them (unless they’re detrimental to my job satisfaction) in getting paid more so that they’re less likely to be recruited away purely for financial reasons. I’d also be concerned about the future projection of my own comp.
However, I'm all on board with the idea that people who do want to exchange this information should be free to do so without punishment from their employer.
Follow-up email: "Thanks for the helpful feedback in the review! And yes, as instructed, I would never dream of talking to co-workers about compensation. I know that's against policy."
Then, either he puts in writing that it's not policy, or you've started a paper trail.
What I've seen work well is the company taking pro-active steps to be transparent about compensation in aggregate. For example, HR can put out a periodic report documenting compensation metrics by job title and experience. They can also slice the data along various social factors to quantitatively show whether there's any indicators of unintentional bias. From there, another good idea is for the company to regularly apply adjustments independent from performance, in order to keep the metrics healthy.