The treatment of PoWs was legally to follow international guidelines. In article 2 of the Geneva Convention from 1929, the rules for how to treat prisoners of war were as follows: “Prisoner of war (…) shall at all times be humanely treated and protected, particularly against acts of violence, from insults and from public curiosity.
Measures of reprisal against them are forbidden.
Germany was responsible for the prisoners of war in Norway. The German government had committed themselves to the international arrangements. However, the Soviet Union had not approved (ratified) the convention and the Soviet government felt they were not bound by any international agreements in their treatment of prisoners of war. As a consequence, German authorities refused Soviet prisoners of war the right to receive any help from the Red Cross.
The Geneva Convention from 1929 made it clear that the states had to give prisoners a humane treatment. But the agreement did not specify what this actually meant. It only stated that prisoners of war were to be treated with compassion. There was no decision to improve the convention until after the Second World War.