Retail order flow has a price because knowing that your counterparty is, almost by definition, the dumbest of the dumb money is worth a non-zero value. Imagine being an auto mechanic or an HVAC repairman and being able to choose only the most gullible low-IQ customers, paying a token amount for the privilege.
slightly different because the way these firms earn isn’t on screwing the retail, but by facilitating the flow.