And they will cannibalize your profits, up to 60%, if you start to make significant money. And they can kick you out anytime they want. It's all in the terms and conditions.
IMO the premium charge is much better than commission as a pricing model. It's less predictable day to day but long term it better aligns incentives between the winning customers and the exchange. They don't really kick people off unless they're exploiting bugs in the exchange or engaged in illegal activity. They certainly won't ban you for winning on the exchange.