It seems to me that the OP completely shifted the burden from the outset without real justification.
A fighting game should have realistic, intuitive controls. A human can stand with their feet planted, raise their arms to guard their face and torso, and not move backwards. A human can walk backwards without raising their arms to shield their upper body. Unless one lives among weird, uncoordinated people, the fact of the matter is that realistically the bio-mechanic action of blocking and the bio-mechanic action of moving backwards can be done completely separate from each other.
Yeah, I know we are talking about games where people shoot fireballs and levitate and all that, but still, even in a game with fantastical moves, who wouldn't want the fundamental controls to correspond intuitively to human motor function? Walking backwards. Blocking. These are pretty core actions in a fight. If we watch a kickboxing match they punch, they kick, they block, and they move around.
Thus, it seems the real question is: "why the firetruck would someone designing a game intentionally link the backwards motion to blocking?" Not every game does this. It isn't like there aren't enough buttons. Console controllers have plenty of buttons. Really, it seems like if we were designing a game from scratch, no prior knowledge of other video games, just trying to make a cool fighting game, the intuitive sense would be to have different controls for the raise-my-arms-up action versus the move-my-legs-back action. If not only because different limbs are involved.
Since blocking is a fundamental part of fighting and something a person can do quite independently of moving backwards, it really seems the onus for justification would fall upon those who would maintain that there shouldn't be the natural separation. Those who suggest a dedicated blocking button shouldn't exist really bear the burden, IMHO.
