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).
There's certainly not an unlimited amount to spend on salaries; as long as a company is not willing to spend 250k on everyone, but is willing to spend it on some, this could be true.
Ethical, sure. Wise? I don't think there's anything malicious here, but the idea that a potential employer would see me announcing their comp strategy in a public forum would be enough to make me wary about posting about it.