We're doing this via a flag so that we can control this behavior should there be sufficient outcry.
oOh dear lord, that's a horrible idea. You make a change to your software to fix a problem that is not caused _by_ your software? If a form is confusing enough that the user thinks they have focus when they don't and ends up losing data then that's an issue with the form, isn't it? Not the browser and not the button.