"Don't we already" is an excuse, not an argument.

There were food and drug products that were unsafe, “adulterated” and mislabeled. This created health problems in the United States so we created the FDA to regulate food and drugs.

Do you have any argument why they shouldn’t do this?

Toxicity and fraudulent misadvertisement is a far cry from sugar being bad for you if you choose to eat too much of it.

And to reiterate my point, advocating X based on "don't we already Y" instead of the actual merits of X is a big part of how dangerous slippery slopes occur, even if Y was a good thing.

In this case, the poster said:

"Education, public awareness, these kind of initiatives I can get behind. Regulation of food intake, is a step too far."

I merely pointed out that the thing identified as "a step too far" already exists. I wasn't advocating for anything so there isn't a slippery slope to fear.