If you generate money for them, they index you, even if the content is bad, and vice versa.
If Pinterest is a buyer of Google's ad services, Google has a strong interest in ensuring that Pinterest doesn't get free organic results. More organic results, less need for ad spend.
Answer: Yes
The use of Wikipedia's information in search results is generally useful to the user.
Pinterest's spamming of search results can harm the user experience, making it harder to find content that is hosted elsewhere. Often the content elsewhere is better than that found on Pinterest because it is more likely to be the original source (or at least cite the original).
I find the Resurrect Pages firefox extension very useful for a lot of paywalls.
I'm not sure that Pinterest's mission is the same.