DuckDuckGo (the search engine) is doing the same. E.g., search for something that returns Amazon search result, and the link to Amazon will have their affiliate tag included.
It is not really the same as DDG is providing the links itself while pinterest is modifying user submitted links. However, I have no problem with either of them. It would be nice of they said it in their FAQ (or wherever) as DDG did.
A positive point, however, is that they are open about this on their website: http://help.duckduckgo.com/customer/portal/articles/216405-m...