According to the safeway article linked above:
Safeway's plan capitalizes on two key insights gained in 2005. The first is that 70% of all health-care costs are the direct result of behavior. The second insight, which is well understood by the providers of health care, is that 74% of all costs are confined to four chronic conditions (cardiovascular disease, cancer, diabetes and obesity). Furthermore, 80% of cardiovascular disease and diabetes is preventable, 60% of cancers are preventable, and more than 90% of obesity is preventable.
They probably know what they are talking about, seeing as how their premiums have remained stable over 4 years while the average increased 38%.