Most people who use Pinterest are there for the cool things they can find and re-pin. Many of those find and pint new items themselves, adding to the experience overall. Lots of people have expressed dismay that someone might want to use Pinterest for anything commercial, while other see no problem with it so long as they don't wind up feeling "sold to" every time they visit the site.
Some items pinned on Pinterest are for sale. Not necessarily directly by the pinner, but somewhere. And, Pinterest is taking advantage of this to make money. If an item is available through an affiliate marketing arrangement, Pinterest uses a program called SkimLinks to add their unique identifier to the link, thus generating a commission for themselves if you buy the item.
Most people don't have a problem with this. If they choose to buy something they found on Pinterest, they are fine with the site getting a commission. But, the issue some have with the practice is that Pinterest never told them this was happening.
Other sites do this. For example, if you use the Metafilter community website and post a link to a book on Amazon, Metafilter will place their Amazon affiliate ID within that link. But, they tell you it will be done. Everyone understands this and expects it. Apparently, Pinterest told no one. So, if a pinner happens to work through a program that has an affiliate element within it, and spends the week pinning up interesting items that are for sale, they have no idea that Pinterest is adding its ID link to all their pins and that htye could end up with a commission payout for all their personal efforts.
Again, most people are fine with this if they know about it. But, it is bad form for Pinterest to not disclose it.