At least with DuckDuckGo I know that it's searching for what I told it to, rather than searching for something easier to find, which seems similar to a machine, but completely different to me.
Dog Cat
Will return results for Dog OR Cat
To force to AND one has to quote every word, thus:
"Dog" "Cat"
Which is tedious especially on mobile.
I don't know why they think people would want to default to OR.
However, on Google you do get an identifier next to the result that shows whether your term is missing from the page or not which allows for weeding out "unrelated" results relatively quickly. It also has the "must include" link so that you can narrow your searches without having to retype the query.
Doing the same search on DDG just shows some results but it's not clear whether my specific terms are on the page or not.
So I believe that there is a consensus that using IP addresses to improve search results would be fair game for DDG. It's also consistent with the DDG promise: "we don't track you".
OTOH, not using them has only marginal effect on user's privacy: (a) DDG doesn't serve ads anyway, and (b) the metadata of user's online activity are still available to three-letter agencies to analyze.
Also IP by itself isn't really that much of a privacy problem. It only becomes a problem when it's used in the process of delinkage of more sensitive information
Bringing all the result pretending everyone lives in US sounds like a crappy strategy.