As a long time employee of the Canadian banking industry (and also a long time hiring manager) - yes this is completely normal and completely legal. Per your offer letter, the job would have been offered to you conditionally pending the outcome of a full background check. I assume you signed/agreed to the offer letter. This means that you accepted the terms of the offer letter (including the background check). You will find the hiring procedures are extremely similar across all the big Canadian banks (banks outsource these checks to external firms who are specialists in this field).
I agree that establishing good credit in Canada is probably a very good next step. I don't believe that hiding your credit history from the States is possible since I believe the background check asks for past addresses and other personal information related to residency - and if you are not truthful about your past, you will run into a host of new problems...
Are you in a position to go back to the individual who rescinded the job offer to ask what minimum credit qualifications you would have to meet in order to be considered for the role again? This would give you a target to aim for. Good luck!
