If its free then You are the product.
Why would certain unnamed hedgies on Dearborn Ave. want to buy a bunch of piker orderflow?
Because they front-run and B-book the shit out of them. They have 599 SEC violations on the books for front-running. Don't be so naïve.
Fully disclosed and custoemrs are getting free commission to buy and hold a stock which is the purpose. Who cares if you get the stock $0.05 higher price, you didn't pay a commission. If they were doing something illegal they would have been shut down years ago.
If you buy 150 shares and pay 5 cents a share higher price it "costs" you $7.50. If you are long term buy and hold then what is the difference if you got to accumulate stocks over time and pay almost nothing to do so.
Paying for order flow is LEGAL.... say it with me now.... LEGAL.