Think it's because in theory no ice can form below -40C SAT, so you could have AI off at all times in these circumstances, but Boeing recommends AI on during descent regardless as it's gonna get warmer. As to AI on below +10C TAT, thats because the TAT will vary with IAS at a constant SAT so there is no SAT at which AI on would be appropriate. My guess anyway.
Cheers
Fat Dog