I've always disagreed that sagging pants originated from prison culture. This is my theory, and I mean it seriously: When Michael Jordan came to the NBA, he wore his University of North Carolina shorts under his Chicago Bulls shorts during games. This required him to wear much larger, longer shorts than other NBA players at the time.
Jordan became an immediate, transcendent superstar, and in the process of starting a fashion trend of basketball players wearing shorts that to went to the knee or below, as opposed to the shorts that stopped mid-thigh for everyone else.
Few people had the money to go out and buy several pairs of longer shorts, but they still wanted their shorts to go to their knees, so they started "sagging" their shorts.
Waistbands didn't start out where they are now, below the rear end. It was a gradual, incremental evolution. Over time, the shorts (and pants) got lower and lower.
This is my theory, and I think it is valid. The jail-house theory is espoused by those who are ignorant of sports, and Jordan's influence on culture.