This, exactly. It's worth noting that this is one of a few income streams that take advantage of retail investor appetites -- another is when when retail broke

This, exactly. It's worth noting that this is one of a few income streams that take advantage of retail investor appetites -- another is when when retail brokers who do "smart routing" of orders lease a feed with advance look at those orders to HFT firms. That kind of income is generally NOT returned to investors, and at some retail brokerages can make up 80% of profits. It's true that if something is free you are the product... but even when you're paying you may be getting sold on the side.