Why do "gangstas" spit to the left when I drive through their "hood."?

What's the meaning of this unhygenic and low-cl act of expectorating saliva whenever a law enforcement officer is kind enough to patrol their streets? Where did they learn this odd behavior? Why spit to the left and not the right? Is it an act of defiance and disrespect, or some form of OCD peculiar to their subculture? As a cop, graduate student, and future professor, I must get to the root of this anthropological mystery!