Couple thoughts from someone who burns a lot of firewood.

1) If they paid $100/face cord (2' x 4' x 8' row), that would still be 200,000 cords of wood. It takes about 15-20 cord per winter in the coldest climates to heat for a season. Keeping the math simple and saying 20 cord per winter, that is enough wood to heat 10,000 homes or small buildings (barracks?) for one season.

2) I doubt it GREATLY if they are transporting this stuff anywhere. Transporting firewood from region to region is very dangerous because you end up introducing beatles, bugs, and diseases to the new area (like ash bore beatle and dutch elm disease)

3) Maybe they are just buying it from locals to "win hearts and minds" or some nonsense like that. I doubt the afgans sell a lot else the US military wants or needs (besides opium)

4) Or, maybe they relize that when SHTF, they won't be getting much fuel oil to heat with...