Why I did I say that the Taliban were not behind Wali's assassination? Because Sardar was not a member of the Taliban movement. Also, he didn't carry the same ideology as the Taliban. That is why he was involved with Wali for close to a decade.
1) They wanted to score some credit with the international community, demonstrating that they had dedicated their lives to help NATO.
2) They knew the real reason, but didn't want spread shame on their family.