Why doesn't tumblr allow Amazon affiliates links?

I posted an affiliates Amazon link on one of my tumblr blogs. However, it seems that the affiliates code has been stripped out of the link in both the dashboard as well as the blog post itself, so that the link is now just a barebones Amazon link. If I go back to edit the post in plain text mode, the full link is still there in the html, but it doesn't show up anywhere else.

The link is correct in the plain text editor, and should be:
http://www.amazon.com/gp/product/0865...

But the link in the dashboard and on the website is:
http://amazon.com/o/ASIN/0865716307

Is this a policy of tumblr's? No affiliates links allowed? And if so, may I ask why?