They've openly said they were selling order flow since the very beginning, so I can sort-of understand the downvotes.
Read any second post by Matt Levine over at Bloomberg as to why selling of order flow isn't necessarily bad, and probably quite ok, for retail customers. But the gist is: fulfilling retail customer orders is lucrative for market-makers because they do not run the risk of fulfilling some order that is part of a larger, institutional, trade that will make the market move against them a short time later.
Read any second post by Matt Levine over at Bloomberg as to why selling of order flow isn't necessarily bad, and probably quite ok, for retail customers. But the gist is: fulfilling retail customer orders is lucrative for market-makers because they do not run the risk of fulfilling some order that is part of a larger, institutional, trade that will make the market move against them a short time later.