Naked shorting is illegal because it allows manipulators a chance to force stock prices down without regard for normal stock supply/demand patterns.
When naked short selling occurs an individual agrees to sell a stock that they neither own nor have borrowed. Subsequently, the failure to deliver creates what are called "phantom shares" in the market which may dilute the price of the underlying stock.
In 2008, the Securities and Exchange Commission (SEC) amended Regulation SHO to further limit possibilities for naked shorting by removing loopholes that existed for some broker/dealers
SEC Fails-to-Deliver Data http://www.sec.gov/foia/docs/failsdata.htm Scroll to the bottom of the above page for the link to the text file with NWMT fails to deliver data