Actually, a bank is obligated to report suspicious activity, no warrant is needed. If you walk into a bank in the USA and deposit irregular amounts of money or a large sum of cash, they can and will report you to the Treasury and banking regulators.
You're erroneously assuming that banks can detect all possible suspicious activity on their own
You're right, that's probably not possible to begin with.
However, if a bank spots patterns, which don't smell quite right they are obliged by law to report it to regulators.
Another reason why Deutsche is yet again in so much hot water[1]. Essentially they refused to report suspicious activities, which was flagged by a compliance officer.