Market makers have always taken a small profit for providing liquidity. The HFT machines take a much smaller profit than the humans who used to do this by hand

Market makers have always taken a small profit for providing liquidity. The HFT machines take a much smaller profit than the humans who used to do this by hand because, like in many many other places in the economy, computers are cheaper than humans. It's the same sort of automation that we've seen in many many other places.