There is risk to the bank for bad-credit people having an account.
1) CSR interactions cost them way more than they'll ever make from the account, if the person stays poor forever
2) A lot of banking actions are not proper "transactions" or atomic in the CS sense -- you can end up overdrawing an account pretty easily. A genuinely poor person is eventually going to get charged off on this.
A lot of poor people fail chexsystems (bank account credit check, basically) from bounced checks, and thus can't open accounts. Which then limits their access to the banking system, including receiving funds, forcing them to use more expensive alternatives.