I just had a quick look at Twitter's terms, and it seems pretty clear that tweets are not Twitter's data. The copyright remains with the poster, and Twitter has a non-exclusive licence.
So while I agree with your basic point that starting a business so completely dependent on something that could disappear overnight is probably unwise, it seems it's really only the API that is Twitter's, not the content itself. That could be a significant distinction, depending on how this all plays out.
that seems like a fairly meaningless semantic point. twitter controls the access to the data, therefore it is their data. all the copyright means is that tweet authors are free to take the content of their tweets and put it somewhere else.
Nobody is entitled to twitter's data except twitter.