In general, citadel wants to pay to trade with retail investors because it knows it isn't going to face adverse selection. So it will give them tighter bid/ask ratios (this is better for the customer) than they would get if they were trading in the open market, citadel isn't going to get hosed by one of them (because there's no adverse selection)
In general, citadel wants to pay to trade with retail investors because it knows it isn't going to face adverse selection. So it will give them tighter bid/ask ratios (this is better for the customer) than they would get if they were trading in the open market, citadel isn't going to get hosed by one of them (because there's no adverse selection)
It's win win win