Leaving aside the fact that BMI isn't a great metric (as mentioned in another comment); we can probably come up with a better one.
80% of healthcare costs are driven by 15 conditions, which in turn are mostly affected by 8 behaviors [1]. I bet that the bulk of the cost is due to completely changeable lifestyle behaviors (diet, drinking, smoking, physical activity).
Alcohol and tobacco should be more heavily taxed (fast-food is trickier, as we need to make sure that there's healthy, equally-accessible food options out there). Education around healthy lifestyle choices should be more available, as well as gym memberships/fitness equipment/trainers. Sure they do cost money, but we're already paying so much through non-preventive healthcare.
Any other ideas how to disincentivized unhealthy lifestyle choices?
[1] http://www.aon.com/attachments/human-capital-consulting/2012...