Not defending Xiaomi in general, but it's worth mentioning that the stock calculator in MIUI (at least when I last used it) was much more than just a traditional calculator. It had all kinds of sophisticated functionality that goes beyond our arithmetic, such as currency conversion, which obviously requires network and an api that might very well be third party and require a privacy policy.

So while I assume they're tracking users, I don't think the calculator having a privacy policy is as shocking as it initially sounds.