Bad analogy. HFT is not like a supermarket. HFT would rather be the guys at the local supermarket running behind you and right in the moment you're about to grab something off the shelves they grab it buy it and put it back on the shelves so you can buy it for a higher price only. And they'd be allowed to do so because they own parts of the supermarket. Oh those guys don't exist at supermarkets? Well funny...