And that is how I would consider the relationship between commissar and tech-priest. The commissar is not qualified to really question the tech-priest and wouldn't intervene unless faced with a clear breach of allowable conduct...
The problem is that if he isn't who is? You can't just have Enginseers running around doing whatever because no one save the Admech has them under control. And if it's the commander that has that right then there's no way he'd accomplish the job in a sane manner, he'd have to give out the ability for others to have a minor amount of authority over them.