Why was Arnold Schwarzenegger not allowed to dub himself for the German dub of The Terminator?

.. and despite the fact he can speak German, the actor did not provide the voice-over for the German release. Instead, German voice actor Thomas Dannebeeg provided the German translations of "I'll be back". Schwarzenegger wanted to dub his own voice for German versions of the Terminator films. However, his accent is considered very rural by German/Austrian standards. As a result, it was decided it would come across as unfitting and rather bizarre to have a gun-wielding robot from the future come back in time and sound underclass and colloquial.