Andrew Left, one of Wall Street’s best-known short sellers, is facing years in prison after being convicted of securities fraud. Should investors be concerned?The prosecution’s case was that Left would take both short and long positions, publish commentary aimed at moving the market, and then rapidly exit those trades once the price moved in his favour. They also alleged he shared upcoming research with hedge funds in advance, receiving a cut of their profits, while failing to disclose those arrangements.A defiant Left says the verdict is a “sad day for free speech” and could discourage investors from expressing honest views about companies.This columnist has often defended short sellers, an unpopular bunch who nevertheless help root out fraud (think Enron, Wirecard and others) and expose weak and over-hyped companies (think dotcom stocks in 2000, and Anglo-Irish Bank and other financial stocks in 2008). Left himself deserves credit in this regard, particularly for his exposé of drug company Valeant Pharmaceuticals in 2015.However, complaints about “free speech” and the value of short selling miss the point. Conventional long investing is obviously a good thing, but pumping and dumping stocks isn’t. By the same token, short selling is all very well, but short-and-distort schemes are not.It leaves a bad taste in the mouth if commentary is used less as genuine analysis than as a way of trading on the reactions it provokes – in effect, “taking candy from a baby”, as one of Left’s messages put it. In cases like this, there is a thin line between opinion and exploitation.