I live in California, and have done extensive research on this - Sates differ greatly though.
My answer to Health net HMO in So Cal is, yes it does, stating on my contract "when deemed medically necessary". I have just seen my bariatric surgeon and he states this usually means BMI>40 or a BMI>35 with 2 co-existing diseases (weight related), such as diabetes 2, or high blood pressure.
Ultimately, I think it may depend on your contract with them and the coverage you pay for.