Economics, qua social science, is not a normative field. But much of the drive to understand how social interaction works is to give advice about policy. However, giving advice implies a standard for determining what counts as good advice, some kind of value theory. This is inconvenient for economists, who want badly to make policy recommendations, but who tend not to be very sophisticated moral philosophers (though there are some notable exceptions). Bryan Caplan tries to find a way around the inconvenience:
In many cases, there is no need to state your moral premise, because (economics + almost any moral premise) will do.
Suppose legalizing the market in human organs would make sick people healthy and poor people rich. What moral premise would imply “don't legalize”? Sheer malevolence? Blind adoration of the status quo? While these are coherent moral premises, they're so rare that the cost of addressing them is a waste of time.
It seems that Bryan thinks most opposition to markets in organs is a function of either ignorance of the likely consequences or perverse and exotic moral premises. This makes me wonder if he has ever debated this issue with anyone? Lots of people understand the economics well enough but continue to believe that markets in organs ought to be illegal. Here's rough sketch of the standard argument.
Human beings have a certain dignity that is central to the value of human life. That dignity ought to be respected, preserved, and protected. Allowing the sale of human body parts diminishes the dignity of those involved in the transaction and erodes respect for the dignity of human beings generally. Therefore, markets in body parts ought to be legally prohibited.
Is this a good argument? No. I think it's lousy argument, even in its most sophisticated form. But the idea that the value and conditions of human dignity imply that some things shouldn't be bought and sold is not at all rare. Indeed, I think this is likely the dominant moral stance of most people in most places at most times in human history. If one grants the benefits of legalizing markets in organs, which I certainly do, then addressing this argument is not only not a waste of time, but is of fundamental importance in removing one of the main barriers to great improvements in human health and well-being.
Which is just to say, no, you can't get around defending your moral premises by claiming that once the facts are established, all moral premises worth taking seriously point in the same direction. It's just not true that there are “many cases” in which all paths converge like this. And when there is such a case, the convergence is often counterintuitive, and thus needs to be demonstrated, not just asserted. Policy analysis is at least as much applied moral philosophy as applied economics. Without some normative standard, economics has no application at all. Moreover, public deliberation about policy requires taking other people's moral beliefs seriously and you can't do that by ignoring them.