eishet yefat to'ar - the nature of halachic ethics

Parshat Ki Teitzei opens with the parsha of eishet yefat to’ar – a beautiful women taken captive by a Jewish solider during battle, who subsequently becomes permitted for him to marry. The Torah seems to acknowledge that the temptation for the solider is too great – “lo dibra Torah elah k’neged yetzer hara”, the Torah allowed the permissibility of the yefat to’ar as a concession to the evil urge (Kiddushin 21), but by placing the parsha of ben sorer u’moreh immediately afterwards warns that no good will come out of such a relationship (Sanhedrin 107). How is one to understand this idea? If indeed, the yefat to’ar is ethically wrong, then how can the Torah allow a concession to evil? And if indeed it is morally justified in battle to claim a captive wife, then why should no good emerge from such a relationship?
The Shiurei Da’as (vol 2 "Bein Yisrael l'Amim) poses the following chakira: are the laws of the Torah a reflection of what is right and good based on the design of nature, and punishment and reward are just natural outcomes of violating the rules, or are the laws of Torah like the mandates of a King, arbitrarily set up based on certain objectives but distinct from the natural order? Is G-d like a doctor who tells us how to live in harmony with nature, or like a King who establishes a social order above and beyond the base dictates of nature?
Like most chakiros, both sides have a grain of truth. The gemara (chagiga 3) tells us that the mitzvah of hakhel includes bringing children so that their parents would receive reward. We certainly have many mitzvos we can do to receive reward – why do we need this extra detail just to pile on more schar? The Sheuiri Da’as explains that parents will naturally be forced to bring their children along with them even without a mitzvah, but by adding a tzivuy, the Torah transforms natural consequences into a mitzvah. It is like the King ordering someone to take his/her vitamins – you would do so anyway to be healthy, but now you reap the extra benefit of showing obedience and gaining reward for the effort. The chiddush of hakhel is not that Hashem arbitrarily creates mitzvos to give us more schar, but the fact that where there already exists a predisposition for an action, Hashem will sometimes add a tzivuy to what we would naturally do anyway in order to enable us to receive reward.
The yefat to’ar is the other side of the coin. A solider who engages in taking a yefat to’ar is violating the natural order and therefore no good can come of his relationship. It is like placing your hand on a hot stove – the burn is an inevitable natural consequence of the act. However, unlike hakhel where the Torah added the mandate of mitzvah to what already is part of the process of nature, here “dibra Torah k’neges yetzer hara”, the Torah made an allowance for man’s base nature and did not legislate a prohibition. The danger of the ben sorer u'moreh outcome remains , but the Torah adds no other barrier to the soliders actions.
There are other examples R’ Bloch brings to illustrate his point (maybe I’ll get to a few more later) – it is worth seeing the entire essay. For those familiar with R’ Ahron Lichtenstein’s essay on whether there exists an ethic independent of halacha, it seems to me that the underlying issue is the same.