Bandwidth is cheap nowadays, few are the webmasters enabling anti-hotlinking to save bandwidth.
Frequently, the issue is that while you may want your pictures on Pinterest, but that doesn't mean you want them on every other website, unbeknownst to you, or even on your competitors' website. You can't file a DMCA if someone hotlinks an image you've sacrificed to Pinterest's use, off Pinterest's servers. The hotlinker's host has no jurisdiction if the hotlinked image is hosted on Pinterest. Ultimately you'd have to remove such an image from Pinterest, or nothing.
As always, there is the issue of scale.
If you have 1000 images on your website and 3 of them are pinned, who cares, really.
If you have 1000 images on your website and they are pinned 30,000 times, that's another can of worms. In some cases, that renders the source website... obsolete.