I'd also point out that it can be more simple than that - what you've described is 100% arbitrage. However, perhaps someone simply buys several levels (sweeps)

I'd also point out that it can be more simple than that - what you've described is 100% arbitrage. However, perhaps someone simply buys several levels (sweeps) the BATS book. Assuming they didn't also take out ARCA and IWM and everywhere else, it then becomes a race to buy at those exchanges. Sure that example won't give a risk-free profit, but getting good edge like that thousands of times a day will average out to a nearly guaranteed winning day.