I've heard free speech isn't free, and Brandon Duncan, who raps as Tiny Doo, has learned that the hard way.

Until recently, Mr. Duncan spent eight months in jail on "gang conspiracy" charges arising from several shootings in San Diego from May 2013 to February 2014. Prosecutors admit he wasn't at the scene of the crimes, and they have no evidence linking him to the shootings. Mr. Duncan, who has no criminal record, also says he had no knowledge of the crimes. But the district attorney charged him all the same because he raps about shootings.

That's not only absurd; it's a blatant violation of the First Amendment.

Mr. Duncan, after all, is an artist, whose music reflects what he's seen. "I'm just painting a picture of urban street life," he recently told CNN's Don Lemon. "The studio's my canvas …. I'm not telling anyone to go out and kill somebody or go do something. I'm not doing anything differently than [Grammy-winning rapper] The Game ."

But the San Diego district attorney thinks otherwise. Under an untested law , an "active participant" in a "criminal street gang" who "willfully promotes, furthers, assists, or benefits from" any felony committed by fellow gang members can be charged with "conspiracy to commit that felony." Unlike traditional conspiracy, the charge doesn't require any agreement to commit the crime. Instead, it requires either "promoting, furthering, or assisting" the crime, which means being a direct accomplice, or knowingly "benefiting" from the crime.

Whether or not this law can be used to prosecute others, the prosecution is abusing it to charge Mr. Duncan. To "promote, further, or assist" means to aid and abet, and there's no evidence he did that. That leaves "benefit." The district attorney alleges he has "benefited" from shootings committed by unidentified gang members through making a CD called " No Safety ," on which he raps about shootings, and through receiving "praise" for his music. As the prosecutor admitted, he wouldn't be charged if he sang "love songs."

That's wrong on so many levels that it's hard to know where to start.

This is a textbook case of using a "gang" dragnet to over-criminalize people of color. It's not even clear Mr. Duncan is an "active participant" in the gang under California law. But even if he is, the prosecution's theory violates the First Amendment by punishing him for the content of his speech. The government can punish crime, but it can't punish speech about crime, even by proven criminals about their own crimes, much less an artist like Mr. Duncan.

In striking down a "Son of Sam" law , the Supreme Court held that the government can't sequester income from speech based on its content, even speech about crime, because that would punish the speech itself. Under that principle, the government can't prosecute Mr. Duncan for singing about crime or receiving "benefits" for doing so. Indeed, this case is even worse, because it involves criminal prosecution, not just loss of income.

On the state's theory, it could prosecute a current or former gang member for writing a book , giving an interview to an author , or appearing in a documentary about gang life , as well as a substance abuse counselor who draws on his gang experiences , because they all "benefit" from knowledge of crimes committed by gang members.

It's all preposterous.

The state can't criminalize protected speech, and it can't criminalize Mr. Duncan's music. Thankfully, Mr. Duncan is now free on bail, but he still faces "gang conspiracy" charges, which threaten a potential life sentence. The ACLU has filed a brief asking the court to dismiss the charges immediately.

In criminal cases, however, the process itself is often the punishment, even without a conviction. Mr. Duncan spent eight months in jail when he never should have been arrested in the first place. The court should stop this case in its tracks and send a clear message that prosecution for protected speech cannot be tolerated.

Free speech may not be free, but it's not a crime.