Why do people trust in what a corporation running closed source software says? And I'm not talking about google, I'm talking about duckduckgo. As far as I know ddg might run by NSA personel and in NSAs servers. Just think about it, whats better than having companies give you some data? You being the company harvesting the data. My point is. Companies should earn our trust showing how and why we should trust them with our data being open about everything they run and do with it.
DDG has a decent about page and privacy page which does somewhat answer your questions. But at the end of the day they're still a company, so there's always an element of trust that they will do as they say involved (whether you're using Google, DuckDuckGo, Startpage, Disconnect.me or some other search engine).