They're not saying that, no, not at all. The scientists merely present the facts, the correlations, the incidences. It's up to people like me to point out obvious stuff such as the fact that being drunk/stoned/high can make a person incompetent to care for an infant, or that a person with psychological challenges is more likely to use/abuse substances. The leading theory about SIDS in the medical community seems to be that all infants occasionally stop breathing, and if they are face down in a bed, or have compromised lungs, they are more likely to stop and not start again. If there is not an attentive caregiver around to notice, the kid's a goner. If the mother has the infant slung on her chest and is watching it all the time, odds of SIDS are ridiculously low.