> If my diet is the cause of a medical issue, then I'm the one at fault.

This makes sense for drugs with strong correlations to specific medical issues. Alcohol usage -> liver failure. Smoking cigarettes -> lung cancer. Trying to generalize this to entire categories of food is farcical, because most food isn't bad by itself unless you eat too much of it without eating other things. And if insurance companies are given the power to set premiums based on diet, they certainly aren't going to burn money and effort trying to tease apart the nutritional nuances of every possible thing you could eat. They'll instead be incentivized to do the "easy" thing and impose blanket dietary restrictions under their insurance plans, and use any violation as an excuse to deny claims or increase premiums.