Because Obama, that's why! (straw man, I know, I know...)
Dude, seriously... My suspicion comes from a drastic difference in policy between this administration and the previous with specific respect to playing nicely with foreign nations. I think (and I'm pretty sure that's still ok) that this administration could see holding the book burners accountable (criminally?) as a way of appeasing Karzai and various Arab leaders throughout the region why the collective we try to figure out how to leave gracefully.
Squorch - it might be helpful for you remember that this forum models itself after a ready room where rules of evidence and the scientific method are not required for the exchange of ideas, opinions, and thoughts.

). Shocking that it didn't happen! From a purely political point of view to discipline the folks that did this more than just a slap on the wrist, sternly worded letter or a talking to would be a disaster here in the US. That alone would give any political leadership serious pause no matter the party.