I am presuming this is quasi-legal but in some cases not? What makes it illegal? The HFTs firms pay for order flow and that is by a certain instrument? So if there is an HFT Bot making the market in certain options at a certain strike my order may go there faster than a different strike for example? Or the market maker Bot makes a certain range of strike prices? Anyway, I am not so puzzled by the legal aspects of this rather the ramifications for the market and possibly taxing the HFT traders out of business. That might bode poorly for a lot of people.