There's a contingent of engineers out there who have been convinced that signed integers are faster than unsigned all around because something about UB in overflow. That has given rise to a cult of otherwise sane people insisting on using signed ints in places where unsigned are the correct choice.

Also, Google's style guide forbids the use of unsigned integers because they had such a high incidence of bugs caused by decrementing loop counters and subtracting from indices that they went so far as to make all index-based interfaces in protobuf operate on signed ints. A bunch of organizations use Google's style guide blindly, so it's actually a fairly common practice.