Where does this outlook on step-parents come from?

Anyone who has answered one of my questions before, knows that I have asked quite a few about my stepkids. I have done this because only a few of my friends and family have stepkids, and none are actually raising them, so I have been looking for life experience and advice from other stepparents. However, I have gotten alot of nasty comments, basically making me out to be a wicked stepmom, an intruder into these kids lives, and an uncaring person, none of which is true. I have only given about 25% of the entire story and have been judged very harshly on that small bit. One thing that I have heard from alot of people on here, something that none of my family or friends agree's with, is that step parents should never disapline. Where does that view come from? And if a stepparent has taken on a full time parenting role, grocery shoppings, school clothes, parent teacher meetings, etc, why should they be treated as a maid, and not truely as a parent? They are doing the duties of a parent.