A very good distinction. But even when it comes to internal comitment, that internal commitment can also be because of wrong reasons, such as respecting principles a certain person has. You have also quoted something about scientific integrity and principles. Such principles are a wrong reason to take a course of action and they are based on idealism, so even internal commitment can be because of wrong reasons. From a Buddhist point of view, we should adopt a course of action based on pragmatism. One of the 3 fetters removed by a stream enterer is rigid observance of rules and vows.
So what pragmatic reason is there for not consuming alcohol ? The starting point is how this world works and how to escape it. We can not eliminate craving by just wanting to elimitate craving. We need to follow a series of steps and gradually achieve the goal. For example it is difficult to eliminate craving by living in a mansion full of woman and cocaine. Even if we have really do want to follow the path and reduce craving, it will be difficult to do it in bad circumstances, such as in a mansion full of woman an cocaine. It is much more difficult to resist indulging in pleasure when we are in close proximity to it. This is why the monk lifestyle was invented. A person will follow the path much more easily if he finds himself in certain conditions.
We need to be pragmatic about it. We may say “I am going to eliminate craving even in this huge mansion full of woman and cocaine” but that is simply much more difficult to do. We need to follow the path by respecting the laws of physics and pragmatism.
It is because of this that indulgence in alcohol should be gradually reduced. It reduces mindfulness and develops bad tendencies for the mind. It is detrimental to the path, to the series of steps we need to take in order to achieve the goal.
Buddha knew people are inclined to respect rules out of rigidity and out of narcissism, to feel superior to other persons not respecting such strict rules. But even in such a case, respectance of those rules is still beneficial to the person. Buddha, pragmatic as always, created these 5 percepts as a way to help lay people who did not understand why these 5 percepts were even created. The rules for bhikkhus also serve the same purpose: to force the person out of narcissism, shame, conformism, whatever - to respect some rules that are beneficial for him, even if he does not do it out of pragmatism and out of proper understanding of the path. Even without following them out of pragmatism, they are still beneficial. And even a person who understands the path might still use narcissism, conformism, etc to his advantage. If he is not an arahant, such pressures exist in him even to a small degree. And he can let them help him in respecting rules out of rigidity, therefore helping him achieve the goal.