didn't the customers notice that they were given bad execution prices ? or would that be very hard to notice in practice ? (I'm not a trading expert obviously)
It's also a fairly victimless crime, as the article notes (although it downplays this); in practice, it seems users typically saved more money thanks to Robinhood's zero-fee trading than they lost in less optimal pricing for their trades. The problem here wasn't that payment-for-order-flow is bad — even Schwab does it for their zero-fee trading platform, and in practice it's often a good tradeoff — the problem is Robinhood lied about it.
RH demanded 80% of it for themselves. Sure, their customers still got a better price than available in the lit exchange(s) but they were bilked by the 4x hidden commission.