The Kelly criterion provides the optimum bet size given the chance of winning. The adversarial brokerage could use the Kelly criterion and only make a wager com

The Kelly criterion provides the optimum bet size given the chance of winning. The adversarial brokerage could use the Kelly criterion and only make a wager commensurate with the likelihood of the trader's failure. It should be profitable. Suppose the brokerage just puts 5% of their cash toward this. It could be a nice profit center without risking everything.