The U.S. Department of the Treasury announced this week that it will take an extraordinary step: it will begin tracking the so-called secret buyers who invest in ultra-luxe properties behind the screen of shell companies. Starting with Manhattan and Miami-Dade County—two of the hottest markets in the country—Treasury will require real-estate companies to disclose the names of buyers who make all-cash purchases or conduct transactions through limited-liability corporations, as The New York Times has reported.

The decision follows a February report in the Times that looked closely at some of the shell companies involved in the highest-end New York real-estate purchases and found that some involved foreign buyers who were caught up in various illegal enterprises, from corruption to organized crime. This rule would force them to identify themselves to real-estate companies and require real-estate companies to report them to Treasury.

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In certain U.S. cities—especially New York, but not exclusively New York—high-end real estate will continue to serve as a powerful tax shelter, even after Treasury’s decision to pour disinfectant on the industry. It will still be perfectly legal, for example, to invest in ungodly expensive real estate and avoid taxes. That’s how it works in New York: Billionaires don’t pay property taxes at anywhere close to the levels that non-wealthy residents do. And when mega-wealthy buyers decline to occupy these properties as their primary residences, they can avoid paying income taxes, too.

What remains to be seen is how many of these buyers today are shadowy foreign billionaires as opposed to regular foreign billionaires or even plain old domestic billionaires. Many less-than-nefarious buyers also use LLCs to conduct real-estate purchases, if for no other reason that the privacy it affords. With the shell option closed to them, presumably very wealthy buyers will . . . continue to buy real estate in New York. So the Treasury rule may stop bad guys from buying lofts on Billionaires’ Row to skirt taxes, but it won’t prevent merely rich guys from doing the same.