I can explain exactly where they’re coming from and why they are “shocked” by this.
The base assumption underlying all “liberalism” is that Man is basically good and only behaves in negative ways (not eating healthy) because of external influences (like vegetable availability).
Since this is an ASSUMPTION that is the BASIS of their belief system, it is not subject to examination.
And they indeed are SHOCKED that people don’t respond to the changing environment in the way that they logically think they should, based on this assumption.
That has been one of the main themes of liberals/leftists for as long as they've been around. In their view, the common person is just a helpless pawn in the great scheme of things easily manipulated by gigantic, evil i.e. capitalist forces who make them act and think the way the evil capitalists want them to act and think. Nobody is responsible for their actions in lib world....unless, of course, they're the evil capitalists.