Although aspects of this sort of separation go much further back. Being from "the wrong side of the tracks" is an expression that probably dates to the 19th century. (Of course, that was less of an actual physical barrier.)
In the case of East Palo Alto, the disparity can be traced back to at least the prohibition era (1919). EPA was the openly "wet" town during prohibition (after the now forgotten city of Mayview's speakeasies were shutdown), and I speculate the behavioral norm to overlook laws, no matter how misguided, fostered an environment where other undesirable business ventures thrived.
Another way to view it would be that speakeasies thrived where the police didn't bother (or accepted discouragement) to go. Police resources are financial and political, and often stretched thin over poor or minority neighborhoods. So it is strange to divorce something and call it just "norms" when such strong economic forces are in play.