Its really frustrating not being to go back using the back button, it hampers the general flow by which I use most of the websites. Its already 2015 here, why does bank websites not allow back buttons?
Banks are extremely slow movers, probably for somewhat valid reasons since every little change must be examined and tested in the extreme for defects, vulnerabilities and regulatory compliance.
Adding that it actually takes a fair amount of work to make browser back/forward buttons work in a web application, it is really no wonder that the situation is what it is.
My bank immediately logs me out if I hit the back button (instead of using their "go back" link) at any time during a logged-in session. That's whether I'm making a payment, looking at my statements, setting up direct-debits etc, or just sitting ambiently on the home-screen. It's incredibly annoying.
You won't get charged twice on any well-written site, but I'm still careful with the back button around any transaction.
It's not really an issue at either of my banks, because a transaction is a lot more than merely submitting a form, and requires authorization with a single-use code. But webshops tend to have far more linear transactions, and I don't always fully trust them to use the back button the way I think it should work.
Adding that it actually takes a fair amount of work to make browser back/forward buttons work in a web application, it is really no wonder that the situation is what it is.