I've posted on forums in the past on same subject about system fleets, but alas you still see them all over the galaxy. Alliance leaders should take the lead and inform all new players joining them.
It's not just the new players either..... A top 10 player had a fleet up blocking a black hole for me today!! Unfortunately he had too high a cloak to make it worthwhile for me to try and see or remove it!
well, f you're in the top ten it helps, cause you usually can't hurt them enough, but everyone else should be wary of placing those fleets.