What they did was traded between themselves with different brokerage accounts, to create offsetting positions. One broker then gave rebates for creating liquidity and the other didn't charge any for taking it. Zero risk, while pocketing the rebates.
This doesn't make sense to me. Why is the amount of risk taken a concern of the government? If I understand correctly, the source brokerage was offering rebates for trades, with the hope that the trades were providing liquidity. The trades did not provide liquidity. The brokerage's hope would seem to be at fault here.