Sure, they could. Would that be any different from how a bank could steal funds from a traditional deposit account?
By making a bank the custodian of your crypto wallet, you're placing your trust in them and should have similar legal recourse you would have had with a fiat deposit.
By making a bank the custodian of your crypto wallet, you're placing your trust in them and should have similar legal recourse you would have had with a fiat deposit.