I thought it was put to prevent cheating in ongoing contests, but I am unable to figure out what utility it is of when put on questions from a contest, for which knowing answers in the present is inconsequential, is asked.
Wouldn't it be enough if OP simply gave the source of the question?
One plausible explanation is that there is a sub population of MSE users who are exclusively browsing to ask and answer exclusively contest questions. If the policy is done for them, then I think it would be better to not put entrance questions in there because the style of entrance questions vs that of contest questions (eg: Olympiads) is very different.
For instance, in most olmypiad participated by highschooler, the emphasis is on Euclidean Geometry. However in these entrance exams, this is rarely the focus. The requirement is usually more holistic. So, it would be a disservice to those who are searching for the normal idea of olympiad questions.