The example of the letter to the editor that you gave (”sheep love”) is an extreme one.
Let’s go back to the example of the person writing about a safety issue.
In that example, he wrote to the paper, put his position and company and then was let go.
Then he spoke out some more, telling the press he was let go due to having previously written a letter to the paper about a safety issue.
So, that employee doesn’t work for the company any more- the contract, as you put it, is broken, so on what basis can the company go after him further?
That further action on the part of the company is retaliation on a whistleblower.
He was stating facts, and any retaliation for whistleblowing only makes the company look worse.
Since when is telling the truth slander?
Well they can still go after them civilly, but if the company is wrong they lose (legal fees etc, defendant can counter sue for legal fees) and the safety issue is remedied thru regulation enforcement (fines, etc), or in severe cases criminal prosecution.
If the employee is falsely accusing due to sour grapes, etc., the same civil suit applies, but the company wins.