I'll tell you one thing, employers love having company minded individuals who'll restrain from telling how much they make because it makes it much harder for us to decide whether or not we're getting a reasonable deal. And you can be fed much easier the propaganda which consists in saying we should cut pilot pay because we're expensive and times are tough.... for one thing, we're all much cheaper than the workforce 10 years ago since we're flying more, sometimes much more....
My 2 cent on why I think it's ok to say how much you earn.