Doing so does carry quite a bit of political risk. There have been quite a few lawsuits from EFF and ACLU in regard to do so, and as the comment from CEO of Duck Duck Go says in the comment thread, all existing cases has been about turning over records. Going the extra step of compelling people to install hardware and keeping the operation going would be a further step.
I doubt ddg is currently worth the political risk. There is likely much easier targets to attack first in order to get 100% of the worlds search data.
*down votes? Explanation?