That’s not precisely accurate.
Generally we’ll serve them a production order, not a warrant. Tough to go in and physically search a bank and come out with records you’re seeking. A production order compels a third party who possesses records and who is not a suspect themselves to hand those records over. A Justice of the Peace must be convinced by a police officer that there’s reasonable grounds to believe the bank possesses specific records that will afford evidence of an offence.
Banks do a number of things that may not be in the clients’ best interests, because the law/lawyers say they have to. Banks are subject to numerous laws, in particular the Proceeds of Crime (Money Laundering) and Terrorist Financing Act (PCMLFTA). This is the act that empowers FINTRAC. Banks are one of a number of entities who have to report suspicious transactions, or transactions over a specific threshold, to FINTRAC. This might end up shared with police. If a customer is laundering money it’s definitely not in their best interest for the bank to report that, but the bank will generally still do so- if they notice the suspicious activity. Sheer volume of banking activity means they often don’t until we start sniffing and they discover a customer may be dirty.
So yes, banks guard customer privacy fiercely- but will also obey the law.