The other problem is that the predictor for HIV is risky sexual behavior. Circumcision is not correlated with reduced risky behavior.
EDIT: the downthread comment clarified the parent comment for me.
As I said even further downstream, if the study was done naively, where we simply took circumcised vs. uncircumcised people and compared their HIV rates, I would agree 100%.
But the original study was taking a population of adult uncircumcised males in africa and circumcising 50% of them randomly, and then tracking their HIV infection rates. That should control for upbringing. From the studies:
"In these studies, men who had been randomly assigned to the circumcision group had a 60% (South Africa), 53% (Kenya), and 51% (Uganda) lower incidence of HIV infection compared with men assigned to the wait-list group to be circumcised at the end of the study. In all three studies, a small number of men who had been assigned to be circumcised did not undergo the procedure; likewise, a small number of men assigned to the control groups did undergo circumcision. When the data were reanalyzed to account for these occurrences, men who had been circumcised had a 76% (South Africa), 60% (Kenya), and 55% (Uganda) reduction in risk for HIV infection compared with those who were not circumcised."
Under this interpretation, being conservative is the cause of both circumcision and low hiv rates, and circumcision is negatively correlated with hiv rates.
It was, "We're going to take a chunk of individuals in Africa who engage in risky sex, and circumcise half of them, and then track how many of them end up with HIV after a few years"
If someone is engaging in risky sexual behavior, and they will continue to do so, then the 10 year span of whether or not they get HIV may be irrelevant since the disease itself is incurable and ultimately fatal because they will continue to expose themselves to the risk factor until they join the infected population.
It's also highly questionable if it would even control an epidemic in Africa effectively, seeing as how the spread is caused by misinformation in a huge part - a widespread campaign of male circumcision would likely increase the exact behavior which spreads it.