This is a common method of payment in Finland as well, but, the store always asks you to choose your bank, then redirects you to the online bank where you then authenticate and authorize the transfer on the bank's website. It then redirects you back to the merchant's website.

Basically, banks provide an API (similar to OAuth? I guess) where the merchant only asks the bank for a specific amount of money, and the authentication and authorization of the payment happens only directly between the customer and the bank.

Why would this be so hard to so in other countries?