中止 literally means to stop halfway (中で止まる). 中止 in Chinese also only means to terminate something already started. How did 中止 get the meaning of cancelling something not started at all in Japanese?
As aguijonazo pointed out, when the Japanese use the term 中止 for an event that has not yet opened, it means that all the planning, preparation, and processes that were already started to make the event happen have come to an abrupt end. This makes perfect sense, at least from a Japanese perspective; for most events, the duration of the preparation and promotion of the event is much longer than the actual duration of the event itself (which is typically only a few hours or days).
So it's merely a matter of what is regarded as the "start". I've never even heard the opinion from a Japanese speaker that we shouldn't call it a 中止 because it hasn't officially opened yet. Such an opinion sounds fairly strange and cruel to a Japanese speaker. (I don't know the specific history of when this discrepancy originated in history, but it's not at all surprising, is it?)