I would categorize this as more of a catalyst than a cause. Once the breakdown of family norms begins, this would accelerate the process by putting more young black men into prison, turning some of them into absentee fathers, thus normalizing single motherhood. However, strong family structures tend to discourage illegal or risky behaviors like drug dealing or drug use, leading me to believe that something else must have happened to weaken them before large numbers of black men would end up in prison for drug offenses. Furthermore, I doubt if there is much overlap between the set of "guys who would deal illegal drugs for a living" and the set of "guys who would settle down with the girls they impregnated and be good fathers, if only they weren't in prison." In other words, the increase in drug dealing/use (which facilitated increased incarceration due to draconian drug laws) and the increase in single motherhood were not cause and effect, but rather effects of the same root cause.

The problem with the Rockefeller drug laws and subsequent enforcement was the unequal application - despite the fact that whites and blacks use and deal drugs at roughly the same rates, blacks are more likely to be arrested and jailed for drug crimes and given longer sentences.

You talk about "norms" but as was pointed out in another HN thread (http://news.ycombinator.com/item?id=2031655) drug use is essentially a "norm" in the USA. The problem is that only certain segments of the population get punished for it, which leads to not finishing school, not being able to get a job, etc. It certainly is a cause.