I can get these results for a site search, coop, but only if I start from the Google homepage (or [google.com...]
OK, just did the same using "site" in the search and did indeed get the results. However, as you mentioned, do it from the advanced search page and it is a different story. For me at least.
What I can't quite figure out is that when I request the AJAX-y URL, live HTTP headers records a request for the traditional URL with a question mark. Maybe Google are requesting the whole page via AJAX and then just switching the content, but that would be slightly strange.