I consider it a flaw in the web-browser to allow those keys to be overridden.

The first URL returns an HTTP redirect to another page; so when you press back, you go back to the URL which immediately redirects you to another page.

I've wondered for a while why browsers don't catch this and let you back-button over the redirect. Perhaps there are some cases where it would be the wrong thing in an annoying way. It is annoying when I can't back button. For me, neither OSX Chrome nor OSX Firefox skips the URL with the redirect, both of them leave the back button not working well. (You can still hold down to get the back history menu, to skip over the redirect)

It looks like it works fine with Chrome's back button. But if I use the back button in Firefox, it always takes me back to the index.

Also happens in Chrome on Android.