Affiliate links could make Pinterest an insane amount of money. Did you know that when someone clicks an Amazon affiliate link, everything they buy in a 24 hour period the affiliate gets a cut of?
There are a LOT of people using Pinterest, and a LOT of Amazon products on pinterest.
An "insane amount of money" - care to be a lot more specific? That's about as vague and unsubstantiated as you could get.
1) The assumption is that the affiliate link model and market will scale to the size of Pinterest's eventual traffic maximums, that's a big assumption.
2) The assumption is that Amazon will want to allow Pinterest to suck profit out the door at that scale, instead of Amazon competing with Pinterest in one form or another instead, just like they're now competing with Groupon / Living Social (despite an investment in LS).
3) Amazon has a very long history of directly competing with companies that touch their platform.
That's an extraordinarily shaky business model to say the least.
There are a LOT of people using Pinterest, and a LOT of Amazon products on pinterest.