They certainly do it for organic results sometimes too. A/B testing or measuring quality of results? Annoying to no end if you want to copy an url directly. I remember looking into it a few years ago and they even went so far as to add js setting the status bar text to the normal url on mouseover.
Even if we redirect users to the search result sites, we don't track by the url, but track by number of hits and number of subscriptions as the performance indicators.
