This is so interesting - one wonders if this is strictly immoral. I propose this thought experiment: What if there was a version of this fraudulent brokerage that conducted this behavior in the open? Let's say their policy is something like, "You trade for real under favorable commissions and margins. If we decide via internal algorithms that you are liable to lose money, we will instead pocket your trade and credit…
That's basically taking a short position on whatever the customer thinks they're buying, right? Whenever the customer wants to sell their position, you'd have to pay them whatever the value was at that time, whether the position had gone up or down. Big risk, unless you're confident that the customers are going to reliably make terrible trades, on balance. It might still be fraud maybe if you claimed to be performing…
There are arguments that naked shorting should be allowed, but it isn't allowed right now.