I think the reason it's a reportable offense to blizzard is because rooftops are used to evade NPC guards who are there to stop PvP in certain areas.

A quick google search pulled up this (old) thread with this interesting quote:

It is additionally important to note that our stance toward rooftop camping in Azeroth has not changed and will remain a punishable offense when verified by a Game Master.
Seems pretty clear to me that attacking anyone from a rooftop in any non-outlands area is a no-no regardless of how much "fun" you have doing it or how long you did it for.