The short answer for if placing affiliate marketing links in Pinterest is legal is NO. Is it ethical? That’s up for debate. You can however place a link in pinterest to your website which has an affiliate link.
I’m not a lawyer, but i’m pretty sure it is against Pinterest’s terms to place affiliate links in there by the fact that they are taking the time to remove those. But, what is what Pinterest doing even legal? From what I remember, you aren’t allowed to stake someone’s content and post it on your site without an agreement INCLUDING images. Isn’t this EXACTLY what Pinterest is doing everyday by taking other people’s images and using them on their own site for monetary gain? They are directly taking images from other sites and reproducing them on their site. There is a little leeway where you are allowed to turn an image into a thumbnail and use that, but the internet laws are really pretty muddled.
Unspoken Internet Law
In short I believe the one internet law should be:
DON’T COMPLAIN ABOUT YOUR COMPETITOR STEALING YOUR IDEAS AND CONTENT.
It happens with even the biggest of companies. I remember back in 2005 Google came out with their revolutionary map panning ability. Prior to this we had to click stupid arrows to reload the page and pan the map. Within 2 years ALL map websites had this ability.
I don’t want to advocate anything illegal, but Pinterest is making money off their user clicks as well. Affiliate marketer on Pinterest aren’t forcing anyone to follow them or buy their products, they are simply working within the system to advertise their products in the most advantageous ways. And Pinterest happens to be the perfect platform for doing so.