I dunno. I’m not suggesting that what they do is illegal, just that it’s useless profit skimming from the inherent inefficiencies of humans getting together to make deals. Before high speed trading, it would just be noise in the line that would level out within minutes or even seconds, but these guys can tell their computers to trade pretty much instantly, and make money off banal shit like “mutual fund X starts their daily 401k purchases right after lunch, and it takes them Y minutes to finish them, so if you see Z orders coming in, have the computer trade at certain threshold and make a few cents.

I’m making that scenario up, and they’re constantly competing with regulators and other firms so it might not even be viable, but the granularity and timeframe and how stupid it is to be able to reliably make money off that kind of variance is very real. Humanity doesn’t move that fast, and there’s nothing of value to society from doing a blackjack rake like that. It’s a fairly small part of the overall market, but it’s just money and brainpower being sucked out of the system for no reason whatsoever.