"The answer is simple. It is inconvenient for our employers. They cannot afford for their workers to compare pay stubs, it might reveal how arbitrary and illogical the criteria are for determining salaries. In many cases, it might even reveal how grossly unfair they are.
...
You can tell, or not tell, anyone you want. Just don't let anyone else dictate to you who you can or can't tell. THAT is none of THEIR business."

Your answer may be simple, but I don't see how an employer would stop an employee from telling anyone how much money they make. Though providing medical coverage, retirement options, and advancement opportunities are a means of obscuring how much you make, but I doubt you were referring to this.

The more simple and obvious answer involves envy. Envy is more important because their are more serious consequences associated with it.

One can turn it around and look at what is said about how much someone says they buy something for.
Some people will brag about what a low price they paid for something. And the same person could brag about how expensive something was. Basically they bragging about their prowess as a buyer- a urban hunter. The skills showcased may involve their haggling ability, their inside information, and/or their perception and knowledge of detecting a good price. Of course some people could consider low brow to discuss such matters. And it is certainly wrong to discuss such thing in various social settings.