I did not know the extension of Data-Mining that Twitter is allowing (for a fee).
Any user has access to a few weeks of tweets, but Twitter archives years of data that could be accessed and analyzed by such Data-Miners as DataSift.
I searched about DataSift, and yes it does have a deal with Twitter, nothing is said to be unethical, however, even if Twitter’s users should be aware of what could be done with their Tweets, I think it just should be reminded to all.
Keep in mind that whatever you tweet could comeback and haunt you!
A Canadian article by “Globe and Mail” triggered my quest. (No link, but feel free to perform you own search)