This fiduciary disconnect can create a lot of malevolent market behaviour if controls aren't in place. And more often than not, controls aren't in place.
>You win, you get commission. You lose, it's not your money.
You get a wage either way. If the bank profits you get a bonus too even if you lost your customers money. If you screw the bank up the gov bails you out and you still get a bonus.
You win, you get commission. You lose, it's not your money.