The pin option appears if there are images posted using the [img] or [attach] tags. I am working on a solution to make the pin button appear if a post has attachments that are not posted inline (using the mentioned tags).
I disagree with the likelihood increasing of image being stolen if they are pinned. Everyone needs to be fully aware that any content posted anywhere to the
public realm are subject to potential theft or abuse. If you don't want an image to be potentially reproduced in any capacity, don't post it online.
The idea is to leverage the Pinterest social platform as means to expand on social sharing... which is what I assume every poster here is looking for when posting images for the social community to see.