this is incorrect. the sum of n>1 balls each of some non-maximum weight needs not be less than another set including the max-weight one.
when it is further specified that all remaining balls are of equal weight then your solution cannot be optimal since comparing two thirds yields the required information about the remaining third.
First, it is trivial to communicate the problem and any moderately competent person will find a solution within a short time frame. Second, it tests how precisely someone takes a problem definition. (In my experience, the kind of person who glances over such details will also tend to make trivial programming mistakes like off-by-ones.) Third, by varying a well-known question slightly you can easily filter out those who have simply learned the "correct" solution from the internet rather than thinking it through.