How come when someone in your legion alerts a planet, it has to disappear from your database? Shouldn't the planets we alert be kept in the databases of our legionmates. I'm just wondering why this isn't a part of the game. You never hear of a star chart, in sci-fi, that lost the coordinates of a planet just because the ship didn't personally scan it.

Sorry, but I'd rather not keep the planet of a legionmate on my scan. More planets scanned = higher scan power necessary to find new planets.
If the shared planet isn't great, I don't want to have to waste a purger getting rid of it.

Imagine how many planets you would have on your scan log. You would need tons of purgers to get rid of them.

Suddenly, alerting very tiny barrens would become alot of fun.

I think there would be a few more entries into the legion shame list. As for the OP, well I agree it doesn't really make sense, but as pointed out it is in no way practical for them to stay in your log, especially as there's multi abuse and people trolling you by taking an alert for a planet of your, leaving and stealing it.