Blaming a classifieds web site for the actions of an alleged murderer is almost as absurd as blaming high-school pot smokers for September 11. Nonetheless, Craigslist has decided to remove (or at least rename) the “erotic services” category of the site. This from the New York Times:

“Andrew M. Cuomo, New York’s attorney general, said his office had recently notified Craigslist about an impending prostitution case that involved the erotic services category.

‘Rather than work with this office to prevent further abuses, in the middle of the night, Craigslist took unilateral action which we suspect will prove to be half-baked,’ Mr. Cuomo said in a statement.”

Putting aside the obvious hypocrisy of this particular office’s crackdown on this particular brand of consensual human behavior—and putting aside the disturbing implication that our state’s top law enforcement officer does not subscribe to the principle of innocent until proven guilty—just why is prostitution illegal, anyway?

Prostitution will always be a profession, and it may always be a profession more risky than most. But in justifying the current policy, most prostitution prohibitionists make the same type of correlation-causation mistake that the drug prohibitionists make: they assume that the ills that sometimes surround the culture of prostitution—the pimps, the STDs, the robberies, the poor working conditions, and so on—stem naturally from the activity itself.

Yet there is better evidence that the organized crime, violence, and exploitative labor structures are drawn to the industry precisely because it is illegal—and thus outside the bounds of employment law, taxation, legal remedies for fraud, and other forms of regulation.

It’s the same fundamental correlation-causation mistake that’s made again and again by the White House Office of Drug Control Policy and other War on Drugs apologists: the failure to recognize that criminal behavior often arises from black markets just because they’re black markets, not because of what’s being bought, sold, or consumed.

Even the prohibitionist op-ed contributors to the New York Times, in a piece responding to the Eliot Spitzer controversy, can’t avoid making this basic mistake:

“Whose theory is it that prostitution is victimless?…The Emperor’s Club presented itself as an elite escort service. But aside from charging more, it worked like any other prostitution business. The pimps took their 50 percent cut. The Emperor’s Club often required that the women provide sex twice an hour. One woman who was wiretapped indicated that she couldn’t handle that pressure…The transport of women for prostitution was masked by its description as ‘travel dates.’”

Do these authors really think that these working conditions would still be acceptable at brothels if the businesses were regulated under US labor law?

Now, I don’t mean to suggest that the legalization, taxation, and regulation of prostitution—as has been done in Canada and Britain, among many other countries—would change the fact that when sex is sold, the transaction is usually of a certain sadness. In regimes where prostitution is legal and conditions are thus better for women—protection is enforced, wages and benefits guaranteed, and so on—the sadder party would often seem to be the man: he’s just paid hundreds of dollars for a woman to pretend she likes him for an hour.

On the other hand, anyone who assumes that the relationship between prostitute and client is never one of cordiality and good humor probably hasn’t spent much time talking to prostitutes or clients.

The anthropologist Patty Kelly has done so; she spent a year living in a Mexican brothel and studying the industry, and reports in an LA Times Op-Ed (written in the wake of the Spitzer revelation) that, in one of law enforcement’s more spectacular wastes of resources, more than 80,000 people per year are arrested for prostitution-related offenses. Ms. Kelly suggests an alternative solution:

“New Zealand’s 2003 Prostitution Reform Act is perhaps the most progressive response to the complex issue of prostitution. The act not only decriminalizes the practice but seeks to ‘safeguard the human rights of sex workers and protects them from exploitation, promotes the welfare and occupational health and safety of sex workers, is conducive to public health, [and] prohibits the use in prostitution of persons under 18 years of age.’ Furthermore, clients, sex workers and brothel owners bear equal responsibility for minimizing the risks of STD transmission. In 2005, a client was convicted of violating the act by slipping his condom off during sex.”

Imagine that.

So why isn’t US policy informed by the lessons of Prohibition?

One theory is that while lawmakers do sometimes seem to learn from our country’s mistakes, actually drawing analogies from those mistakes is a more elusive feat—as it is for law enforcement agencies, whose extraordinary leeway in choosing what and what not to pursue gives them a power to shape de facto law more than most citizens recognize.

That is, Mr. Cuomo is actively choosing to spend his time this way.

But there’s another, more intellectually plausible, explanation for why this is allowed to go on. I still remember, from law school, the famous “dwarf-tossing” debate that stood for the question of whether any consensual behavior between adults should ever be criminal. Those who believe it should tend to rely on the position that the criminal law, beyond merely creating a system of incentives, can also have a so-called “expressive” nature—society’s expression of a norm (in this case a behavior—the sale of sex—of which it disapproves) by codifying that norm in the criminal law.

But even if expressive laws are sometimes justified, they should not be imposed in cases that would result in obviously harmful human outcomes like the spread of STDs, violent robberies, the exploitation of women, or the vast waste of Mr. Cuomo’s resources on the victims of these crimes instead of their perpetrators.

The US prohibition on prostitution is no more justifiable than—and, in fact, strikingly similar to—the Catholic church’s prohibition on the use of condoms. When lawmakers, whatever their honest “expressive” intentions, maintain a public policy that is acknowledged to bring about disease and violence, they are willfully putting their own constituents in harm’s way.