In order for Puck and Oberon's behavior to be inexcusable upon mortals, they would have to be mortals themselves. Human's lives are determined by fate, but their lives are not in the hands of another human. Since Puck and Oberon are not human themselves, they can be considered forces of fate. Technically the two fairies are only guiding the humans in their journey through their lives and helping them to make the right choices. To be held accountable for their actions, Puck and Oberon would have to be humans, not forces of fate. Since they are considered forces of fate, they have reason to meddle in the mortals lives. The events that have happened due to Puck and Oberon now can be excused since they were acting as a force of fate.
Puck and Oberon enjoy being concealed from the mortal eye. That way these two can help the characters, but one have to deal with the trying to be told to 'mind your own business' since no one can see them. Never in the movie or the play, did Titania, Hermia, Lysander, Helena, Demetrius, or Bottom know that they were being influenced by anything but their own decisions. Fate works like the two fairies; they are mysterious so that they are never seen, heard, or acknowledged. When Titania takes the little Indian boy, no one but her and Oberon notice that the little boy has been switched. The switch was made without anyone noticing. And when the baby didn't get switched back it