Let's shy away from the rhetorical question a bit because we already know that for general mobs and npcs that are engaged: training, kill stealing and ninja looting are against the rules. I'll agree that mem blurring and charming should be added for clarification (although that already constitutes raid interference on bigger targets).
For triggered mobs in the kill x to spawn y category (see plane of sky), we have those rules plus a 20minute engage timer or 1 wipe for the raid who triggers.
The problem here seems to be from epic - quest encounters. After one player turns in an item that triggers an event/npc, how long does the player have before another group or raid can engage, if at all? This is different than a normal 'triggered' mob because, for epics, they will usually require hours, if not days, of work in order to get to the point of triggering their mob in the first place; a 20minute timer/1 attempt is by no means fair.
edit:
Off of the top of my head, some examples now would be the druid/ranger epic event in east karana, Ixiblat Fer, Ragefire, the doppleganger in WK.... and down the road would be the triggered versions of Venril Sathir, Faydedar, and Trakanon, to name a few.
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