While it might indeed be useful to have a disgust reaction to candy or video games, the "ugh fields" seem not to be visceral disgust, but visceral, conditioned-pain-induced tendencies to cut off thought.

In my experience, this only ever happens with a specific candy or game. For example, If I lose repeatedly and humiliatedly in an online game, I might develop an ugh field that ultimately prevents me from even thinking about popping on to play it, but the corresponding comfort activity is far more likely to be "play a different game" than "go study some math instead".