That's true if you assume the only things that fly are enemies unless they're identified as friendlies. That's not always the case.

I remember reading an interview with a pilot who flew in the Iraq war after major hostilities were over. He described a situation, iirc, where he got a radar contact. He notified his superiors (can't remember the exact term he used) and they said they no friendlies should be flying there. It was ok for him to shoot it down. Something felt off and decided to try to id the plane first. It turned out to be a civilian plane carrying contractors or dignitaries (can't remember which).

I can try to find the article again if necessary. The main point is that there are some cases where you do need to "ask questions first" (ie. id the plane) and then shoot.