This hack in safari to allow 3rd party cookies by posting was fixed. (Btw, Google also received a hefty fine from the FCC for exploiting this "hack": http://www.theverge.com/2012/7/31/3207388/fcc-approval-google-fine-safari-cookies )
In any case, the reason that they use iframes is because the preferred method for storing data associated with the 3rd-party domain is no longer a 3rd party cookie, but instead localStorage. To access the localStorage of the 3rd-party domain, the javascript code has to be running under a document from that domain, hence why the iframe works, and not a script loaded on the 1st party domain.
The benefits of localStorage vs the cookie is that it's not blocked even when the user requests blocking of 3rd-party cookies. See for example this thread from Firefox development: https://bugzilla.mozilla.org/show_bug.cgi?id=536509 or this article running through the code itself http://log.scalemotion.com/2012/10/how-to-trick-safari-and-set-3rd-party.html