It’s not about drinking and driving, it feels to me like drinking from a paper bag behind the wheel IS probable cause because it’s so culturally expected for the contents to be alcohol.

From my memory, inner city folks tend to hang out on their front steps or on a street corner, somewhere “public”, partly because until recently air conditioning was pretty expensive and we’re talking about low income groups. It’s also just a good spot to hang out imo. Anyway, because the cops were mostly white, it’s was pretty easy to roll through on a hot summer evening and nab anyone outside with an open container. People got pissed, rightly so, and somehow the agreement was reached that concealing the beverage in a paper bag gave plausible deniability, and allowed them to drink on the stoop.