It’s Islamic law: “A woman may not leave the city without her husband or a member of her unmarriageable kin accompanying her, unless the journey is obligatory, like the hajj. It is unlawful for her to travel otherwise, and unlawful for her husband to allow her to.” (Reliance of the Traveller m10.3)
“Taliban ‘behead’ woman,” The Nation (Pakistan), December 27, 2016:
KABUL: A 30 year old woman was beheaded on Monday evening in Sar-e-Pul province of Afghanistan by a group of armed men, local officials said today.
Provincial Governor spokesman Zabiullah Amani, confirmed the incident and said that the armed men were linked with Taliban. The incident took place in the remote village of Latti in Sar-e-Pul.
Amani said that the women was beheaded because she visited the city alone without her husband. Amani said that the victim’s husband is in Iran and they don’t have children….
However the Taliban rejected any involvement.