Comment 4 for bug 1108637

Firefox on Linux also uses Alt+← to go back instead of Backspace. It has the advantage of symmetry with Alt + → for Forward and prevents accidentally going Back a page when all you intended to do was delete the previous character in a form but your focus was on the wrong spot.

With Nautilus 3.6's quick search I think it would be a bit more difficult to have Backspace do two different actions (which is itself non-intuitive).