History obviously must have a sense of irony. This could explain in part the coincidence of the recently announced New York Stock Exchange-Archipelago merger following closely on the heels of the Securities and Exchange Commission’s imposition of its sweeping Regulation NMS, including the trade-through provision.

The so-called trade-through provision of Regulation NMS is a throwback to the old command-and-control days of the 1970s, when regulators thought they knew what was best for every investor.

Specifically, the trade-through rule requires that orders to buy or sell securities be routed to the market with the “best” (i.e., lowest) price – rather than “trading through” that best price to an alternative trading venue. In an April 6 vote, the SEC approved the trade-through rule, with the Republican chairman joining the Democratic commissioners in approving it.

While investors clearly care about price, their preferences are not as one-dimensional as you might gather from the SEC’s rule. Other attributes like anonymity, trade size, and execution speed also factor into an investor’s trading decisions. An investor might be willing to forego the SEC’s “best” price if it means a faster execution or being able to trade anonymously, especially if that anonymity helps him or her to protect a valuable trading strategy, for example.

Now it would be overstating things to suggest that the trade-through rule is all cost and no benefit, though it is mostly that. One redeeming virtue of the rule is that, along with the relentless march of technology, it apparently gave the NYSE the shove it badly needed. In today’s electronic world, “best price” was increasingly less likely to be found on the Big Board’s slower, specialist based system. But in that redemption also lies a fundamental rub with regulation more generally: A given rule like trade-through may be good under one set of circumstances, but such circumstances invariably change. Human beings are funny that way: They change their minds, their circumstances, and their surroundings, leaving regulators constantly playing a game of catch up.

To be sure, forcing everyone onto the same best-price bus substantially eases the regulator’s job, since enforcement simplifies to the directly observable dimension of price. But what about values? What about investor preferences? It is the diversity of investor preferences, at bottom, that are a principal motivating force behind the NYSE-Archipelago merger. Through its merger decision, the NYSE is implicitly acknowledging that the days are numbered for direct human involvement in the vast majority of securities trades. Specialist monopolies on order execution, in short, look destined to go the way of ticker tapes, Wednesday closings, and paper orders.

Fundamentally, the regulatory question boils down to “Who decides?” If I want to stop at 7-Eleven on the way home to buy a gallon of milk and doing so costs my family, say, a dollar more compared to stopping at a full-service but slower grocery store, who’s to say this a bad choice or that it should be banned? (I can think of at least one person who’s to say, but fortunately for my fellow shoppers, she doesn’t work for the SEC of grocery shopping.) The point is that as responsible adults, we get to decide where, when, and under what circumstances to shop for milk, houses, cars, and securities, and we get to decide at what prices we will part with our money. Just as important, our patterns of buying (and selling) taken together determine where we would like economic resources to flow. Regulations like the trade-through rule (though offering some limited benefit) ultimately short-circuit that voluntary resource determination process. Mergers, like the NYSE-Archipelago deal, on the other hand, are a reflection of that process in action. Ironic, isn’t it?

Mr. Cochran is a senior research fellow in regulatory studies at the Mercatus Center at George Mason University, Arlington, Va.