[cjpearson]

That would be front-running and it's illegal. The article you linked explains what PFOF is, but in general there's nothing wrong with market makers fulfilling brokerage orders. It means traders get a better price on their order and the market makers earn a profit.

The concern is that when market makers share that profit with brokerages they encourage brokerages to select the market maker that gives them the biggest cut rather than the one who gives the best price for the trader. Still, the trader will never get a worse price than what's available on the exchange. (That's also illegal.)