On further reflection, how does the Wayback Machine know to stop archiving?

When I encounter a parked domain (manually), the first thing I do is click my Wayback Machine shortcut (javascript:location.href=%22http://web.archive.org/web/*/%22+location.href as provided by bart), and check the most recent archive. Sometimes I get a cached version of the ads portal, but often, I don't. Anecdotal, yes, but it may be worth inquiring how they do it (wayback@archive.org is their publicly available addy).