So why wasn’t Mickelson charged?
The answer is that certain kinds of behavior previously understood to be insider trading are now effectively legal—or at least not prosecutable. More than 20 years after the imprisonment of Ivan Boesky, the infamous arbitrageur, it’s become vastly harder to convict someone for insider trading—the result of several years of legal challenges that handed Wall Street an enormous victory. An appeals court ruling in December 2014 basically legalized the don’t-ask, don’t-tell information-gathering model employed by many hedge funds; it’s now OK to trade on questionable information that one receives secondhand, as long as you don’t know too much about how it was obtained.
via The Don’t Ask, Don’t Tell Guide to Trading on Inside Information – Bloomberg.