Slander is illegal. Proving it is often difficult. Not in Zimmerman's case, however. They already have an admission of guilt! The difficult part is proving damages, but the simple fact that the Blank Punsters put a $10,000 price on his head seems to indicate damage, as well.
I think he cut a deal with the networks. Something along the lines of "If you'll foot all my legal fees, I won't sue you for one billion dollars."