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 a service that you're not performing, but if you actually told your customers that you were doing it (as in the situation you ask about), then I can't see how it could be breaking any laws. And I'm not certain why your customers would even care, and might even prefer it, due to the "get 10% back if you lose everything" clause which you don't get with normal day trading.