Why not just hire him? Unless there's some legal mumbo jumbo related to closing down his services I see no reason to close up shop ... my guesses would be legal, or tax related.
There are any number of financial and organizational reasons, but I think this could have a lot to do with the intellectual property Values of n holds. Both Stikkit and I Want Sandy make heavy use of natural language processing to turn short pieces of text into structured data, which has intriguing possibilities for extending Twitter's feature set.
A founder is unlikely to abandon his startup (especially one that took venture funding.) And you don't want them to be working on a different company on the side or focusing attention elsewhere.
