They kill fellow Muslims because they regard them as having effectively apostatized by working for the Americans. From [1] AP, :

KABUL, Afghanistan — Gunmen ambushed a car carrying Afghan civilians working on a remote U.S. military base in eastern Afghanistan and killed eight of them execution-style, police said Friday.

The ambush victims, who worked for the U.S. military as laborers in the mountainous Korangal area of Kunar province, were killed Thursday while driving home from work, said Abdul Saboor, Kunar's deputy police chief.

Gunmen stopped the workers' car, searched them and took about US$6,000 (euro4,780) before gunning them down, said Salehzai Didar, Kunar's governor. Two workers escaped, he said.

"This was a shocking attack against these poor people," Saboor said.

Saboor did not identify the attackers, other than to describe them as "the enemy."

Al Qaeda and Taliban insurgents operate in eastern Afghan regions bordering Pakistan.