The problem is, you first need to have a US blog. Whatever this means. Do you need to live in the US, that means your Google AdSense account needs to be registered on a US-address? Or is it enough to blog in US English setting, intended for the US only?
Who knows. It's not easy to tell and apparently you can not see if or if not your blog is participating. Only if you write the post about a topic that fits to the ads, you will see this anyway. E.g. if you blog about a product like Motherboards, you could search affiliate links for that topic in that box and showing ads about that in the posting.
So it feels little bit weird, because we won't know if we are allowed to use it or not. And it's again not for all. That is not good. Many of bloggers that do hard work on writing articles, won't get any benefit from this new feature for one reason or another.
Come on, Google, be more transparent. Show if or if not a blog is allowed to use this new feature. And give a guideline what to do to get affiliate links on your blog.