read the articles - post Flash Crash they changed the rules to explicitly bar the practice, but this case alleges that spoofing was also illegal under a different rule.
The PDF was linked from: http://www.wsj.com/articles/u-k-man-arrested-on-charges-tied...
"The case is part of a crackdown by criminal and regulatory authorities on manipulative tactics used by high-speed traders, including spoofing, which was specifically outlawed in the 2010 Dodd-Frank financial overhaul law.
That law was enacted after the flash crash; the CFTC is alleging that Mr. Sarao violated the Commodities Exchange Act, which also prohibits manipulative trading. It is also alleging that he violated the anti-spoofing law for trading he engaged in beginning in 2011."