HFT practitioners can't create volume unless they actually trade. If they actually trade, then they are taking risk, participating in the market, and providing

HFT practitioners can't create volume unless they actually trade. If they actually trade, then they are taking risk, participating in the market, and providing (or removing) liquidity. Volume in the market refers to shares traded, not quotes. The fact that I'm willing to buy 100 shares AAPL at 12:01:00.000 (enter an order) and not willing to do so at 12:01:00.001 (cancel an order) has no influence on market volumes - because a trade never took place. Now, if you're talking about the limit order books changing extremely rapidly, according to different participants understanding of their positions and intentions, then that's true. That's because the order book represents the intersection of supply and demand at a given instant.