None of this is simply "arbitrage" as HFTers want the public to believe. The major brokers are perfectly capable of buying and selling at the best prices across

None of this is simply "arbitrage" as HFTers want the public to believe. The major brokers are perfectly capable of buying and selling at the best prices across any non-darknet market with even rudimentary order routers. What's happening is HFTers are gaining informational advantages and then exploiting them in microseconds. Based on realtime orders, they'll infer that someone wants to buy share abc at price x, deviation y, and then bet on that happening. HFTers can even infer which brokerage firms are placing the orders based on technical data including lag times and data-sharing partnerships.