There are two issues here.
First time problem, Pinterest conducted monetize too early, right from when they are still in Beta. This is unlike other social networks ever do – exclusively focused on improving and expanding features. For example, Twitter and Facebook has experienced a long period of development, attract users before deciding to accept advertising to generate revenue.
Second is the problem how, Pinterest conducted quietly, do not give any notice to users. This is the issue of concern and attention. When you join the Affiliate Program, one of the basics when you promote your aff links that you need to make clear to readers that it is the aff link, the ad link.
Reason by U.S. law (Federal Trade Commission) has specified that any sort of subject content creation which encourages and promotes the buying behavior and thereby gaining financial benefits they need to declare clear. (This is the reason that people use when participating in MMO games aff or Google Adsense, Disclosure Policy should be more items on their website).
Any blogger can always aware and do that, but Pinterest not (yet?)
Another problem is the feeling, the purchase money and benefit the people it’s not very much fun.
Clearly Pinterest are making money on the content of the user.
According to Josh, Pinterest have used the services of Skimlinks to edit / aff link automatically inserted into the Pins contains links related to the products and with its enormous traffic, Pinterest became the largest customer of Skimlinks, bring a large revenue for the company (Skimlink receive 25% of sales from the affiliate link)