This is like saying a 100% determinate chess playing computer shouldn’t look ahead, since it cannot affect its actions. That will result in a bad move. And likewise, just doing what you feel like here will result in smoking, since you (by stipulation) feel like doing that. So it is better to deliberate about it, like the chess computer, and choose both to one box and not to smoke.
This is like saying a 100% determinate chess playing computer shouldn’t look ahead, since it cannot affect its actions. That will result in a bad move. And likewise, just doing what you feel like here will result in smoking, since you (by stipulation) feel like doing that. So it is better to deliberate about it, like the chess computer, and choose both to one box and not to smoke.