I think DDG is a great example of why Google doesn't have a BOSS-like API. It seems pretty clear that DDG is violating the TOS of the Yahoo API by mixing search results. Yahoo seems to be looking the other way (for now), but you can bet that Google would be less forgiving.

There's not a search engine out there that wants to allow you to muck with their relevance algorithm by changing the results, and Google has more to lose from DDG-like activity than it might gain.