To be clear, I'm talking about scraping. I think the sibling commenter is talking about developing competing products via reverse engineering ("clean room implementation"). I am also not a lawyer, so I can only tell you the guidance I received from one for the projects I worked on.
Technically speaking you can scrape data in a legally defensible way if you do not need to accept any terms of service explicitly prohibiting scraping in the course of grabbing the data. The distinction is that browsewrap T&C have plausible deniability, but clickwrap T&C do not. And if you receive a cease and desist order, you abide by it with a mea culpa. This also means you don't scrape so loudly as to be noticed, which has the happy side effect of probably not disrupting the target's service.
But again: The grey areas of ethics are a separate question from legality. Please engage a lawyer for your specific work.
Depends on the jurisdiction. For instance Europe allows reverse engineering for inter compatibility. Terms of services carry little value here (none for this clause that's already covered by a specific law).
However, it would be interesting in the case of bank services. Accessing the account from the customer probably let you initiate a variety of actions like money transfers or loans, it's sensible to argue for limited and controlled inter interoperability.
Also, customers are not allowed to share their credentials and he is in breach of his contract. The account should be considered compromised and be locked.
Technically speaking you can scrape data in a legally defensible way if you do not need to accept any terms of service explicitly prohibiting scraping in the course of grabbing the data. The distinction is that browsewrap T&C have plausible deniability, but clickwrap T&C do not. And if you receive a cease and desist order, you abide by it with a mea culpa. This also means you don't scrape so loudly as to be noticed, which has the happy side effect of probably not disrupting the target's service.
But again: The grey areas of ethics are a separate question from legality. Please engage a lawyer for your specific work.