This pattern keeps re-emerging.
Online payment systems want your bank login details.
Facebook was infamous for scraping your IMAP account for contact information.
etc.
The implications for security are so severe I wouldn't mind if this were illegal, but certainly it should be legal for banks or cell providers to terminate online accounts of people who share their credentials, no matter if - or especially if - they are with other large corporations. How many times has T-Mobile been hacked in the past two years?
If an account holder wanted to download a data export and upload that to another provider I don't really care so much. It's the near mandatory sharing of credentials that is just such a terrible habit to normalize.
And yes, greybeards, we know you've never heard of apartment rental agencies only accepting Venmo for rent.