Marijuana use actually may be more strongly correlated to higher socioeconomic status than lower.

A quick Google search for "drug use by socioeconomic status" turns up a 2012 article titled "Socioeconomic Status and Substance Use Among Young Adults: A Comparison Across Constructs and Drugs" [0] which concludes, in part

  > Findings based on three indicators of family background
  > SES—income, wealth, and parental education—converged in
  > describing unique patterns for smoking and for alcohol and
  > marijuana use among young adults, although functional
  > relationships across SES measures varied. Young adults with
  > the highest family background SES were most prone to alcohol
  > and marijuana use.

[0] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3410945/

EDIT: Change lede.