That's not so obvious; if a norm against disclosing lets employers pay better performers/ people they want/etc more, then the result of disclosing is a flatter pay grid, which benefits those that would be at the low end, but hurts those at the high end.

I don't know or have specific reason to think it's true, but it's one model that's not obviously wrong (if it is, I'm more than happy to hear why).