Does First Amendment protect Brandon Duncan's lyrics; prosecuted for enticing gang activity in San Diego under 2000 law

When is an artist criminally responsible (or in civil cases, too) for violence his rap or works is thought to “inspire”?  I thought there was better First Amendment protection than this.  San Diego County, CA prosecutors are going after Brandon Duncan for supposed connection with up to nine gang-related shootings in San Diego, using a 2000 California law, Proposition 21, which could criminalize any activity with a street gang “with knowledge that its members engage in or have engaged in criminal gang activity.” But Duncan is promoting gang activity with his music?  Here’s a typical story. 

  

This whole thing has a weird structural parallel to Russia’s anti-gay “propaganda” law.
   
A conviction could lead to the California law's overturning in federal court as a "content-based restriction on free speech". 
     

Don Lemon and attorney Midran Charles covered the issue on CNN Tonight.
  

However, the RT article links to a story about a rapper (Denis Cuspert) in Germany who joined ISIS and is using it to recruit jihadists from Britain (Daily Mail, Nov. 2014, here  )