Why not? Because of liability to law suits for breach of the terms of service or copyright violation. Third parties have been sued for scraping in the past.
You seem to have gotten downvoted without comment, so let me chime in. He's proposing a FOSS client. There's no third party here, the client is using this software to breach their own ToS (supposing there's a clause in there by which you're only allowed to access twitter.com from a browser).