Banks aren't required to have in place mechanisms for detecting ponzi schemes. Their fraud detection algorithms are designed to detect people trying to defraud the bank or fraudulently using the accounts of their customers. While their anti-fraud measures may uncover people defrauding the public if the transaction patterns of the account resemble those of money laundering, terrorism financing or other designated financial crimes, such a discovery would be incidental.
That might be the sort of thing that triggers the automatic fraud detectors.
The transaction patterns of the exchanges are much more likely to be flagged by the bank as possible money-laundering activity than possible fraud. The exchanges themselves have algorithms meant to detect possible fraudulent activity.