That very sentence is a relativistic sentence. Think about it. It's like saying "you should not say you should not." The proposition is self contradictory.
In other words, make it against the law for someone to define morality, then that very law itself is based upon morality (some moral belief) that you hold. Therefore, in the process of making the law against defining morality, you are defining morality!
It's basically saying "it's immoral to define what is immoral."
That's contradictory. So you wind up with the very relativism you first meant to avoid.
There are many good books on this subject including "RELATIVISM: FEET FIRMLY PLANTED IN MID AIR" by Dr. Frank Beckwith and Greg Koukl. See also, "CHRISTIAN ETHICS" as well as "CHRISTIAN APOLOGETICS" both by Dr. Norman Geisler. You might also want to check out "THAT'S JUST YOUR INTERPRETATION" by Paul Copan.
It was a joke. Not an obvious one I admit, but that was my intention. Obviously it wouldn't REALLY be illegal. LOL! Just frowned upon.