I've always assumed that the mama saw homosexual activity. That would explain the line: "Mama looked down and spit on the ground every time my name gets mentioned."
In my culture (Italian American,) spitting on the ground like that is just about the worst way that a person can show disgust and revulsion towards someone else. Something as simple as buying drugs wouldn't explain the mother's visceral reaction.
Can you tell that I've spent way too much time thinking about this over the years.