Their studies, while somewhat convincing, are based on a small segment of the population. Anyone can test 100 infants in a supine sleeping position and apply their findings to an entire population of people, that doesn't necessarily make the finding true, or even accurate for that matter. I have twin sons who are 7 weeks old, and not only do they sleep better on their stomachs, but a few weeks ago the older twin was lying on his back and started choking on his own vomit. Had I not been there, he probably could have died from drowning in his own vomit. I think it is up to the parent to decide the best way to position their child while sleeping.
All infants are different and therefore have different needs, and that includes sleep positioning.