I'm in no way suggesting that step parents are not to be respected, but I'm being a realist. If I am a teen and have a new step-dad, while I am not saying I would not respect him, I am saying, I would never consider him an equal to my mother as far as authority over me. If my mother issue some directive I did not agree with (your grounded, you lose your driving privileges, etc) I would not think to go to the step-father to plead my case. My battle would be with my mother. Period. Flip that to the step parent doing that, and I would not hesitate to plead my case as to why he was being un-reasonable.Families where a step-parent is not respected or not allowed to make rules will hurt the children and the adults' relationship on the long run.
That, to me, is real life and with that line of thinking, as the step-parent, I'd want peace and harmony in my home, so any kind of discipline I wanted to dole out would have to be with her blessing. She might be my wife, but her children will always be first.
Its similar to the way I see the in-law relationship. I will never do anything to strain the relationship between my wife and her family. I consider her to be the primary point person in the relationship between our family and them. Similarly, I would not want her taking any action toward my parents that would cause a problem. Let me handle any unpleasantness concerning them.