[JoshTriplett]

> some companies threaten to terminate anyone who reveals his or her salary to another employee.

In many jurisdictions, this is illegal.

[Amezarak]

It is illegal everywhere in the US. Talking about compensation is protected by the National Labor Relations Act.

[st3v3r]

When has something being illegal stopped a company from doing something that could save and/or make it more money?

[lukewink]

When the penalty of a lawsuit outweighs the money gained from such an action.

[st3v3r]

You're forgetting the likelihood that such a lawsuit will be brought. Which isn't very high.