In general, for n in [2..5], it can be stated that "Max n used BS for backspace because Max n-1 used it."
The reason why Max originally used BS (ASCII 8) for backspace is because that is what the OS used to tell it to say. This got changed sometime in the transition from Classic Mac OS to Max OS X (and was probably always different on Windows, which wasn't supported until midway through the Max 4 cycle).
Whether ASCII BS(8) or DEL(127) is more "correct" is a matter for debate. My keyboard has three different keys that delete (Backspace aka Delete on the main Sholes keyboard; Forward Delete in the PgUp/PgDn block; Clear on the numeric keypad). It would be nice to get different values from [key] for these three keys, as each has a different function. So I would vote for 'bug', but there may be different viewpoints to consider.
Granted: in a pinch you can distinguish between the keys based on the Scan code (keycode, 2nd outlet, or the idiosyncratic 4th outlet).