They simply don't understand the consequences of their actions.
Being a mother of 3 myself, when my children try to do "grown up things", it is quite hilarious. They don't see things the same way we do.
However, I am against provocative behaviour in children as it is normalizing over-sexualization and general provocativeness.
I am all for taking a healthy approach to sexuality. It is one thing to teach your children that sex and sexuality are normal and healthy. They need to understand and have all their questions answered so they can make fully-informed, educated decisions when the time comes. They need to understand the worth of their bodies and all the ins and outs of consent (there are so many blurred lines) and they need to know they can be safe in their sexuality.
Teaching children that being provocative is acceptable is another thing altogether. We have a highly sexualized society and children need to understand what is and isn't acceptable.
However, I don't think that the majority of parents who do think that provocative behaviour is cute realize the implications. I think they think that the children don't understand and therefore it is "harmless".