They decided what is ok to eat and sell from a safety stand point. They are not trying to tax perfectly edible and safe foods just because they can be fattening.
This really goes beyond fattening foods. FDA doesn't block everything unsafe, a LOT of unsafe food is out there. They are still deciding what is and what isn't ok to eat. Taxing food wouldn't tell you what you can and can't eat, it just pursuades you to eat better or pay for consuming (and thus demanding manufacturing of) unhealthy food.
The "average" I was talking about has nothing to do with political or religious views or culture/family history or any shit like that. It has to do with the average of being informed about what people are consuming. A lot of American's are in the dark or just couldn't care less. They need to pay for their ignorance and it's effect on others.