(Edit: In an email update, Google clarified to me that they contacted other advertisers as well. I added the bit reading "among others".)

> The Merchant Search ad format does not compete in the
> auction and therefore does not affect other AdWords
> advertisers’ CPCs or Quality Score

The Merchant Search ad sure affects whether the highest-paying advertiser gets top spot.

"The Merchant Search ad format does not compete in the auction"

Compare to:

<<Quality Score is automatically evaluated in the same way for our keywords as it is for any advertiser's keywords. Likewise, the potential to show up in the top spots above the search results is the same for Google's ads as it is for any other. We rely on the AdWords system to let relevancy and usefulness to our users be the driving force behind our ad placement. As such, we do not intentionally try to secure a top position. In fact, we generally aim for a more 'conservative' position.>>
http://adwords.blogspot.com/2006/12/google-advertising-on-adwords.html

The key thing is that the ad is not a part of Google AdWords. Maybe Google should have displayed it in a separate part of the page, although the only people affected are advertisers.