There's no particular connection between our own demo harvester and OAIster -- as a matter of fact, we'll eventually be pulling the plug on the demo harvester (and likely replacing it with a moderated, OxS-specific harvester). And I don't think that OAIster has any sort of policy on not accepting conference content.
I'm guessing that your suggestion that conferences just don't register their site has some truth to it -- conferences are seen as more transitory than journals, and I'm not sure that ensuring long-term visibility and access is the highest priority for most conference managers (something we'd like to see change, of course, and which is large part of why we developed OCS in the first place). We know that there are *far* more OJS journals out there than OCS conferences; and we correspondingly see far fewer OCS conferences
register their site with us than OJS journals
. Again, we do wish that more care was taken to ensure access to scholarly conference research, and hopefully this will be the case in the future.