> Nobody's saying it's all somebody else's fault.

Yes they are.

> The only valid position to hold about this topic is that it's complex and multi-faceted.

Incorrect. That's a valid position, it's certainly not the only one.

> the carb-dependent metabolism they end up with by the time they reach adulthood will most certainly complicate their ability to make healthy choices.

Nonsense, your metabolism is not set in stone, they can simply change their diets and if they don't, no one is to blame but them for their continued unhealthiness.

> Why do you hold a different position regarding sugar?

Sugar isn't remotely comparable to those two; they're restricted to minors because they're lethal drugs, sugar is not.