It's also an easy way to generate a lot of false positives. To be a spammer does not mean to take advantage of affiliate revenue. I would say this method would be too sloppy to use by itself. It would need to be a small part of a larger set of signals.
Edit: DanielBMarkham's comment does a better job of conveying why this would be a bad approach.
You don't, yet when you go to explain how it could be used you include two other signals (came from Twitter, posts many items more than once) in order to make it work. I had originally said "I would say this method would be too sloppy to use by itself. It would need to be a small part of a larger set of signals."
So alright, we aren't on the same page, but you're saying exactly what I just said.
Breaking captcha is easier. Deathbycaptcha, decaptcher - Two of the services that provide clean APIs for the bots and breaks the captchas for you. And on the other end there are even human sitting and cracking the captchas.
It doesn't cost much and for the $2k he earns per day, these captcha breaking service is just drops in the ocean.
http://pinterest.com/source/amzn.to/ has lots more, and they all have the default Twitter avatar which gives them away. Lots of them have terrible or inaccurate descriptions and are in the wrong category.
They aren't picking it up as spam because they aren't trying to stop spam. They will soon, and it will become very difficult to make much smaller amounts of money. Or, if they don't, they will end up in a place where pinterest is the Myspace of pinboards.