Well, personally, I think that the black-and-white thing has to do more with categorization than decision-making.
This experiment specifically looked at the ability to perform two tasks at once. The prefrontal cortex is associated with what we call executive function, which involves a lot of different processes like holding information in your working memory, evaluating consequences, inhibition (controlling your responses), regulating your emotions, and decision-making. What this task was tapping into wasn't switching between two
concepts or options, but switching between two "programs" to execute in terms of action and processing.
In general, our brains like to take the most economical route whenever possible. Categorization is an extremely valuable mental process--it allows us to learn, organize the world in a coherent way, and generalize or infer--but it also means that in order to take shortcuts, we end up creating fairly rigid boundaries between categories. Spending time contemplating every single category you form would take a ton of time and energy. So if our categories are reinforced time and again, we take those boundaries for granted. So, that's why we get stereotypes, racism, sexism, etc. I would claim (controversially) that these prejudices are "natural", since categorization is natural. That does NOT justify them, though. It's why these things aren't just going to go away, and we need to constantly work against prejudice and stereotypes through education no matter what our society is like.
Did that make any sense?