Runic magic

Here's the situation. The description of the magical ward rune states that "on the next attack, the rune will steal a random **positive** effect applied to the attacked enemy unit." However, Berserkers attacking a unit under the control of a Dark Magic spell stole the control effect. But this (control) is not a positive effect. After that, I cast the control spell on the same unit again and gained control of two enemy units.
Can someone who understands the mechanics explain this?