Except that there's no reason it has to be underground. For a long time, instant messaging services tried to block third party clients, but I've never heard of them trying to take legal action. The Twitter TOS claims that "you have to use the Twitter API if you want to reproduce, modify, create derivative works", but that's copyright language, and Twitter does not own the copyright to tweets; only individual tweeters would have standing to sue for copyright infringement (see the recent Craigslist controversy), and I'm not sure such a claim would be valid anyway. There's the CFAA, but... anyway, I'm not a lawyer, but I'd be surprised if Twitter made a first by suing the makers of an unofficial client. In the meantime, dunno if Apple would let it into the App Store, but they could freely sell their client from their home page.
Well, that's my dream. tent.io is nice, but it's not going to replace Twitter anytime soon, and I actively dislike app.net. But we're past the age when third party clients can be effectively blocked by technical means, and I want to see their return.