First, I see no issue with their Terms of Service. That language is 100% cover-your-ass boilerplate, and any site that allows people to upload content will have a similar clause in their ToS. Facebook, YouTube, SoundCloud, etc. all do.
See, e.g., section 6.C of YouTube's ToS: http://www.youtube.com/static?gl=US&template=terms
If you find people are sharing your copyrighted material on Pinterest you should file a DMCA claim with them. That's how the mechanism is designed to work, for better or worse.
Second, when you react viscerally to what Pinterest is doing or enabling, think carefully about your opinion of YouTube. With respect to content, is there a substantive difference between these early days of Pinterest and the early days of YouTube?
The MPAA is probably saying, "See? You don't like it when it happens to you, either."