In Defence of Pinterest Users

What is the difference between sharing a photo on Facebook and sharing one on Pinterest? Other than that the Pinterest one will usually be linked back to the owner of the work, thus driving traffic to their site (not to mention that re-sharing is generally very good free PR).

I don't understand how some photographers and artists can complain that people are Pinning their work, and then go and share the work of others on their Facebook timeline, without crediting the owners! Is this not hypocrisy?