It's a loss leader. Remember, the manufacturer is Mars, Inc. They'd prefer you buy the higher margined product too, because most likely they make that as well.
If, instead, at the beginning of the decision tree it was a 75cent guaranteed snickers bar then it would be a simple, rational decision - do i want to pay 75cents for this? The argument the marketing department makes is that there are are fewer people who rationally want that 75cent snickers bar vs. the people who can be sold on the dream of a 50cent snickers bar (that strangely looks and tastes more like a high-margin cookie).
Happy that I live somewhere where "fast food" includes very very healthy options (like made-that-day rice balls with red beans).