Showing up in an image index (generated by crawlers) is different enough from showing up on Pinterest (curated by humans) that allowing one to make the choices independently is totally reasonable.
discovery is fueled by
* links
* sitemap.xml
who creates links (and sitemaps)? humans (either editorially or programmatically, but at some point humans were involved in the link creation process)defining the pinterest fetching process as "not indexing" or "not crawling" seems a little bit unfair against your friendly neighborhood googlebot.
also if pinterest is "currated by humans" then the pin-tag would be unnecessary. a simple plain text "DO NOT PIN THIS" message in the article/product/image/.. (whatever people pin these days) description would be sufficient.