Corporations are "creatures of the state." They are artificial entities created by government to allow a group of people to do things that may or may not be permissible for individuals.
Accordingly, governments may regulate corporations in any manner seen fit.
Now, the ERROR in your question is that it is not corporations that have a right to absolute privacy; it is INDIVIDUALS that have an absolute right to privacy. INCLUDING FINANCIAL PRIVACY.
Government, specifically the fascist U.S. government forces banks to violate their customers' privacy.
The blame is on the government, not the banks, for violating customer privacy.
Now, we know exactly why the government does that. Governments claim absolute control over your affairs, personal, business, and financial. Those in government feel that they OWN you, that you exist to serve THEM, not that they are there to serve you.