DDG is (largely) a pass-through proxy for search from Bing (both Web and image) and for Russian-language media, Yandex. It maintains a number of specific search bots that mostly provide its intsant-answers content.
(This is based on interpretation of very unclearly worded statements and comments from DDG over the years.)
DDG may not censor results itself, but it inherits the censorship policies of its search providers.
(Disclaimer: I've used DDG as my primary Web search for nearly a decade, and Images since DDG image search came online. I'm strongly critical of DDG's relationship to the Tank Man image censorship issue.)
You make a good point in the difference in active choice but to say duckduckgo doesn't censor results is inaccurate is it does even if by proxy of where it sources those results from. I do feel this is a no-win situation for DDG and I don't think anyone should hold this situation against them as they seem to be caught up in it against their will.