They probably didn't want to publicly announce (particularly for advertisers to get wind of) this major loophole for publishers to chose ads for whatever keyword they liked, whether or not the content reflected the?keyword or not. I first reported it on October 26th.
Major loophole closed in AdSense:
index.html?keyword no longer displays those keyword ads [webmasterworld.com]
While some people were using this legitimately, others were abusing this to have high paying keywords show on pages where there would not be those keyword ads showing otherwise.
I do believe that once the URL with the?keyword is indexed that it will show targeted ads for the page content, but not show ads based upon your?keyword choice, unless this has changed since a month ago when I reported this trick no longer worked.