Why did Vian free Altmark's prisoners but leave the ship?

Why did Vian not capture Altmark as a prize?

Freeing the prisoners was politically defensible; stealing the damaged ship from Norwegian waters would have been much harder to justify.

Taking the vessel from Norwegian internal waters would have deepened an already serious violation of neutrality. Recovering British prisoners offered enough moral and political justification for the operation without turning it into outright seizure of the ship.