The 2020 annual report of Bank of Montreal contained the following excerpt in the "Report of Independent Registered Public Accounting Firm We also have audited, in accordance with the standards of the Public Company Accounting Oversight Board (United States) (PCAOB), the consolidated balance sheets of the Bank as at October 31, 2020 and 2019, the related consolidated statements of income, comprehensive income, changes in equity and cash flows for each of the years in the three-year period ended October 31, 2020, and the related notes (collectively, the consolidated financial statements) and our report dated December 1, 2020, expressed an unqualified opinion on these consolidated financial statements. Why is this a requirement of a Canadian company?

Why is this a requerement of a Canasian corpany