My only point is that all search engines tend to cannibalise the ones they index to a certain degree when they aggregate or present results from their own website. It doesn't make it wrong to do so, or even unethical. I just think it would be nice if some sort of mechanism could be worked out to encourage people to visit and contribute more to the sites that generate the data being used.
DDG makes it quite plain where the data is coming from, and is very open and transparent with what you guys are doing, so please just take what I wrote as a comment and maybe a request, but certainly not a criticism.
Edit: I just reread my comment and I expressed myself rather poorly I think. The sentence "DDG need to learn how to play nicely..." was not at all what I was trying to convey. DDG do play nicely with others, far more so than anyone else. I apologise for that phrasing, that was unintentionally rude.
I didn't take your comment as rude at all and you make valid points. We're continually looking at how best to serve our users, contributors and the wider web while building trust and protecting privacy, and such feedback helps us do that.
For cheat sheets, we either write the information from scratch (as in the tennis cheat sheet) or rely on existing key->value data such as keyboard shortcuts or command parameters. Again, we link to the source in both cases although our cheat sheets often end up featuring improvements or fixes, added by either the author or the community at a later date. They become living, open source resources and are also exposed in our API: https://duckduckgo.com/api
You're right - we're not a content provider, but so far we've found people appreciate saving even a click or a page load to find what they're searching for.