Yes, because that's a scary slippery slope given insurance companies' past history in finding the flimsies possible reasons to exclude issues from coverage. Pretty much every common chronic condition can be attributed to some combination of behavior and genetics, so if insurers are allowed to exclude coverage for health issues that have a behavioral/lifestyle element to them, you should be fully prepared for not just diabetes and heart disease but also most cancers, STDs, and even accidental injury to be subjected to the same behavioral tests. Sorry, your skin cancer treatment isn't covered because it is attributable to spending too much unprotected time in the sun. Surgery to set your broken leg isn't covered because skiing is a high risk activity. Where would it end?
The entire idea of insurance is to pool risk, and allowing exclusions based on individual factors contradicts that idea. Insurers would love to only cover the healthy and cautious, but on a societal level, that would be a disaster.