About the only thing surprising about the buy buttons that have been popping up lately is that we haven't seen them sooner.
I'll summon up my inner Seth Godin to make one point, and to make it quickly.
Virtual travelers don't want to have to make side trips to do something. Whether that "something" is interacting with friends, or booking a restaurant, or watching a video.
Or buying something.
You must support the actions you most want to see the most where your customers actually are, rather than where you want them to be.
And there's no need to get miffed because people are no longer visiting your website, because what you're witnessing is the extension of things that also take place on your website.
Your website is everywhere.
Or it should be.
UPDATE – 5 June 2015
Barbara Starr has discovered a Google patent with the title "Providing a search results document that includes a user interface for performing an action in connection with a web page identified in the search results document" that she describes in a Google+ post.
A figure from the patent showing an in-SERP buy button
This is interesting in that provides clues about how Google might choose to generate a buy button in the search results, but is perhaps more interesting because it describes a methods by which all sorts of action buttons might be included in the SERPs.
UPDATE 2 – 9 June 2015
Jennifer Slegg reports that Bing is now testing calls-to-action at the top of their SERPs, and citing a query for "wynn las vegas" that results in a "Book A Room" button appearing next to the Wynn ad above the organic results. See a pattern here kids?