Image a time before computers, websites, cloud services, and Pinterest. Image when you would have film cameras, Polaroid, and books. Image you put up your photographs in an art gallery.
Someone went to the art gallery and saw your photo on display and fall in love with your photo. Would be stupid for the person to keep coming back to the gallery every few days just to look at your photo again. Also the person could talk to his/her friends about this photo he/she likes so much, but they may not be able to correctly image the photo. Of course, the person could say "Come with me to the gallery and I'll show you." but would his/her friends really have to go?
In most cases, the person would just have to buy a printed copy (poster, postcard, gallery guide booklet, a book you published), so that he/she could always enjoy looking at your photo again and again, and it is a lot easier to show to friends than inviting them to the gallery. Of course, some would just stick a copy of the image into a scrapbook, either glue the postcard in it, or actually cut and paste the image out of the book and into the scrapbook. On the other hand, why not dare to be bold and take a Polaroid photo of your displayed artwork, so they can stick it in the scrapbook.
It's all about memories. Reminding ourselves we saw it and like it.
Pinterest and the likes are somewhat similar. You see an image on the webstie, and you like it so much that you just pin it. That way, you could always look at the images you like, and also show off to your friends what you like.
But of course, as you point out, there is no way of knowing who is just doing that purely out of liking the images, just want an online equivalent of a scrapbook and who is ripping you off by selling your images. You can't blame the innocent ones for what the villain ones are doing.
Hope you tear the villains to pieces and make them pay, but hope you consider being pleased that the innocent ones enjoy your images so much that they just pin them. Not their fault they didn't think to link it to you, or credit it to you.