I agree that HFT don't buy order flow in order to make trades execute better for retail investors. They buy order flow in order to make money. The retail order flow is, as you say, non-toxic. That is, HFT can reasonably expect to make money with the data and not get rolled over. The question then becomes, who is paying HFTs and why. The highly simplified answer is that HFTs are allowed by the regulatory agencies/exchanges to pay themselves a slice of the bid offer spread in return for the liquidity benefit their market making activity provides. How do they do it? By using the retail order flow information, which allows them to "front-run" the order (step in between offers and asks) and provide liquidity. The offeror gets a better fill and the askor gets a better take and the HFT gets a profit and the exchange has vastly higher liquidity and everyone is happy with this type of front-running.