Upwork are handling the money between you and the client. So if they want to take on that role, they should properly insure themselves in the case of fraudulent or bad payments. If you are not allowed to take on payment yourself, the liability for fraud is theirs. If they decide they do not want to insure against any payment problems, the only fair thing to do would be to allow you to reach out to clients directly and ask for bank transfers. It's not fair for them to force you to put yourself into a position to be vulnerable to chargebacks.