I think the all-or-nothing attitude is an attempt by many people to play against their own psychology.
For many people, "I'll just do whatever I feel like each day" is problematic, because the moment right before lunch isn't their brain's best time to make that decision. In that moment, the task at hand looms large in their attention, so they are disproportionately tempted to submit to it and keep working. The benefits of taking the break are somewhere out in the future, they're abstract, and even if you intellectually believe in the benefits, it can be easy to discount them, right there, in that moment. The task at hand is right there, right then, very concrete, no discount.
For such people, all-or-nothing is a way for them to make a decision ahead of time, in a moment of less stress and greater foresight, and perhaps commit themselves to it in a way that takes the decision out of their hands in their weaker moments. They may not come out ahead every day, but if the strategy works, they should come out ahead in the long run.
For many people, "I'll just do whatever I feel like each day" is problematic, because the moment right before lunch isn't their brain's best time to make that decision. In that moment, the task at hand looms large in their attention, so they are disproportionately tempted to submit to it and keep working. The benefits of taking the break are somewhere out in the future, they're abstract, and even if you intellectually believe in the benefits, it can be easy to discount them, right there, in that moment. The task at hand is right there, right then, very concrete, no discount.
For such people, all-or-nothing is a way for them to make a decision ahead of time, in a moment of less stress and greater foresight, and perhaps commit themselves to it in a way that takes the decision out of their hands in their weaker moments. They may not come out ahead every day, but if the strategy works, they should come out ahead in the long run.