1. No, it isn't. It might have been a mistake, he might be in a hurry and didn't see it was a reserved item, and so on;
2. You take a screenshot of the leader saying the loot rules and not mentioning anything reserved;
3. The ninja reports have a very clear set of requirements to make it so there's no room for doubt. If there's doubt, i.e. you didn't provide all the required proof, GMs will disregard the report, even if you consider it "evident";
4. Already addressed in point 2. People do get banned for it, so they manage to gather the required proof just fine. Your problem is you refuse to understand that you are missing proof that isn't optional.