Why doesn't the autobrakes and ground spoilers deploy on the A320 if the speed is below 72 knots?
Not a definite answer: I imagine someone backtracking on a long 4000 m runway might actually taxi at around 71kts? 

If armed, MAX braking will occur once the system detects that ground spoilers are deployed. 
From the FCOM, ground spoilers:


  If the ground spoilers are armed and the speed exceeds 72 kt, the ground spoilers will automatically extend as soon as both thrust levers are reset to idle.


Because of Airbus SOPs by the time you enter the active rwy, your autobrakes are on MAX and spoilers armed. Once armed&amp;active, they will deploy the second the ADIRS will sense any deceleration. With thr lvl retarded to idle, the plane now thinks you are actually rejecting a take off, not knowing you are slowing down at the end of the runway for the 180deg turn to lineup. All hell will break loose as your plane insta-handbrakes on the spot with the activation of spoilers+max brakes.

Incidentally, my company used to have a 30kts speed limit for backtrack which has been recently removed.