1. There are other areas of the law where it is presupposed that people have unchosen obligations to others; the existence and legitimacy of taxation, for example, is predicated on the idea that it's permissible, at least to a limited extent, for the state to coerce people into paying for other people's needs (or at least for the state's own needs, which inevitably involve the interests of people one doesn't personally know or choose to care about). Indeed, the idea that citizenship involves any individual duties at all, as opposed to all individual rights all the time, is predicated on the idea that the state, which is neither a natural community nor a chosen interpersonal circle, has some right to people's assistance in its own upkeep.
As anyone who's ever received cash for their birthday can tell you, money has high utility but requires little personal thought. Taxation works because it reduces society's needs to a single common denominator (money) that can then be used to fund innumerable programs determined by the government. This is fundamentally different from a stranger being compelled to directly help someone else in a specific way, whether in the case of a violent crime or otherwise.
2. I don't really mean the law when I say "organizing principle for society", no, or at least not only the law. The vast majority of interpersonal relationships that people maintain and moral decisions that people make have very little bearing on the law at all, even in a modern and bureaucratic age in which the state's role in directing people's lives is both more visible and more, let's say, stylized than in the bad old days. As Hobbes (not that one) said, "if your friends are contractual, you don't have any". Surely that's even truer of, say, family, townspeople, coreligionists, and other people with whom one has relationships and to whom one has obligations that one doesn't personally choose the way one chooses one's friends?
3. I've conceded a few times in succession now that my desire for an alternative to a Maximum Individual Choice account of interpersonal obligations doesn't mean that such an alternative is currently available. You (and I guess also Vosem) stressing the nonexistence of such an alternative over and over again seems superfluous to me, unless you're trying to convince me that what I currently believe is an unfortunate feature of the current historical era is in fact a universal part of how the world works. And universalizing that sort of thing is just not an idea that appeals to me as much as it appeals to the average Talk Secular US Forum Atlas Elections blogger.
If you're talking about extralegal activities like charity, then what exactly is lacking in our society? Charitable acts aren't exactly frowned upon by any political or social group. I would contend that charity
is an organizing principle of our society when it comes to social relations; it just wouldn't work as an organizing principle in a legal sense. How much of an obligation do you want to see, exactly? And how would you see it enforced?