I'm guessing they are listed in the settings > apps area because only part of the software is installed. Facebook for HTC Sense is also listed, whereas it's not in the applications drawer either. Both Twitter and Facebook have an API Engine listed directly below them in the settings > apps, so it would actually make sense that while Twitter and Facebook are listed, the only thing actually "installed" on the phone is the portion of the software needed by the API in order to integrate them into *other* apps that need to "share".
It was just curious to me because on my OG EVO, those things were also listed in the applications drawer (i.e. I'd have Facebook listed (the "official" app) as well as "Facebook for HTC Sense" and BOTH had settings I could access and change).
I'm sure these things were just added to Sense to be used as "helper" apps, but I would have liked to be able to uninstall them if I wished (i.e. if I wanted to get rid of Twitter or Polaris Office out of the list of "use this to open this" dialog), or change settings to them. [THIS part is SOLVED .. see below post ]
None of these things are a big deal, I just noticed they were there and it was different from my OG EVO.