[plc]

What are you talking about? If a non HFT buyer/seller both want to buy/sell at 20, then they will go to the market and that will happen. In fact, that happens all the time.

This is precisely the point about HFT. If you are a seller, there isn't always a buyer. HFT is there when there ISN'T the other side. The scenario you described is absurd. How often do a buyer / seller want to interact within the same millisecond?

They can of course. HFT does nothing to stop that. Two people can trade in the open market with each other whenever they want. The reason most of the time HFT is involved is HFT gives the best price. That's why it's become so big. They provide the best price to their customer, not attacking some "flaw" in the exchange.