Basically, there are three categories of images, what I am calling the 3 P's:
Personal - Those you take and post,
Promotional - Those you do not mind sharing as it promotes a product you sell (and hope to get sales from, i.e. a picture of a wedding gown), and
Profitable - Those you make money from (sell), i.e. custom graphics or photographs.
Obviously, the first are not issues. The third is what can get you and Pinterest in trouble.
Obviously Pinterest has created a sort of "get out of jail free" card by offering the meta tag nopin, but that is only a half measure. It does not prevent someone from copying and pasting. That leads to "how to protect your image" which has been discussed here over the years.
You may think at this point, Marshall, you're just rambling. Not really. For years people have shared pictures in scrap books and wedding or birthday or baby books without the knowledge of the image owner. You would cut a picture out of a magazine and paste it in the book, have a party and say "look at what I found," the operating principle of Pinterest. The only difference now is it is easier to copy and share on a wider platform than you could with a scarp book, and get "caught" doing it.
If Pinterest wants to show their concern over copyright protection, their nopin tag should state not just that the owner does not want their images shared, but something to the effect "images may be protected by copyright and can not be shared without the owner's explicit permission." This would bring the copyright issue to the forefront, an issue, and law, which the general public is rather ignorant of.
Whether it's Pinterest, Facebook, or any other form of sharing, the attitude towards anything on the internet is that it's free for your personal use. Whether this presumption is borne out of ignorance or what seems to be the mentality that "if it's on the internet, it's mine to use," it is this presumption that needs addressed.