I strongly suspect that at the time they signed the deal (5 years ago), Twitter didn't have the technical capability to resell data (the site itself was going down lots!), so they had to do deal with DataSift and others.

DataSift previously ran TweetMeme, and did have good technical / scaling capabilities.

(I think I read that there was some other scaled technical thing DataSift did for Twitter too, but I can't remember what - so take this comment as a hint rather than facts)