Actually, there are several better questions the first is considering the significant differences between many parts of Africa and the first world (from an anthropological perspective and others) why are these studies being used to push for infant circumcision in the US. Even if we were to say, for example, that there truly was a 50% reduction it is of little practical value to most people outside high prevalence areas. Second, as far as Africa is concerned, why focus limited resources on infants. Rather if you were going to offer the procedure why not only offer it to older teens an adults, those most at risk. In a place were resources are tight for more reasons than just that this is the only ethical approach.
ETA: And why Africa? Because it's cheap and they'll be able to find a lot of volunteers considering they intended on getting circumcised anyway. In return they got a paid, free health care, and some other things too. But also they need to conduct these experiments in a [locations of high prevalence, extreme poverty, and little education] to see results or focus on people at the highest risk. It's sort of like why they fill a lab rat up with more sacuren (sp) then anyone could consume in their lifetime and observe that it causes cancer.
ETA: Fixed 'real dump'