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.

The back button exists, banks can not refuse to allow its use. All they can do is warn against its use.

Have a unique hidden input for every form. If a user goes back, regenerate the form.

If a form is somehow resubmitted, print a nice message saying saying that transaction was already submitted.

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.

I google that and this was my third result.

http://www.gcflearnfree.org/internetsafety/6/print

Topic 6 talks about it. So simple.

I suspect the OP is asking why bank websites do not gracefully handle the back button and prevent the errors mentioned on that page.

Except that you don't get charged twice on Amazon or basically any web merchant when using the back button.

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.