Who do you think took the money on losing short trades? It is either commission or the investors.
One has to accept the fact that so far they have been winning big.
I am with you. They take no risks and want to take 20% of your profits. That is a 20 delta call option free to them, applied to the winner. If they were to take the other side of their customers, then it would make sense that they charge to offset the loss from the winners.
He takes risk. What does the company do to get 20%? If anything he should charge them 20% off his commission if he is successful. They should even be charged 20% of the profits he makes because they can monetize his volume, follow him etc. So he has to charge them, not the other way round.