Military secrets leaking out through flash drives

There have been a number of stories lately about flash drives leaking out of military bases in the Middle East in recent months.  It's not known yet if military personel are selling the drives on the black market or if cleaning staff is stealling the drives, but they're out on the native economy.  And the drives contain a lot of sensitive information, like soldiers personal information and the identity of informants to the military.  No matter how the information is getting out, it's a huge vulnerability for the military in the area.

I'm actually surprised flash drives are even being allowed on a military base.  Many businesses that deal in sensitive information have methods of blocking USB devices, so why isn't our own military taking steps to protect themselves.  Are the flash devices that are being leaked owned privately by the soldiers or are they military issue?  The military is attempting to purchase back as many of the drives as possible, but what are they doing to prevent further leakage of the media?

Flash drives are easily available with 2 gigabytes worth of data storage.  You can fit a lot of Excel spreadsheets or Word documents in that space.  Even more if you're only taking sensitive documents rather than whatever's available.  If the thieves aren't targeting specific data already, they soon will be.