I was thinking about this same thing last week in the shower. The reality is that I find most people pinning items that are not necessarily to be purchased i.e 20 different wedding dresses, ferraris, crazy tattoos. That sparked my apiphany, like yours, that this isn't to create a store front, this is to curate enough data to proverbaly shoot fish in the barrel with targeted advertising.

I know Google made an offer to Pinterest and clearly it wasn't enough. They should have put all of their eggs in this basket because this is the true alternative to FB's power house advertising that is soon to be bolstered by the inevitable search capabilities.