Go read the old posts by Blackguard around the time KoS came out.
It is by design, and as stated, it is AAXP that is impacted by this. It is there because level 70s would have very few options for gaining AAXP otherwise except for endless XP grinding. It’s also there to give level 70s some incentive to mentor lower level characters to help with quests. It's no different than getting AAXP from killing a named mob that you mentored low enough to make it con green.
The only reason it’s a particularly easy path to AAXP for *me* is that I have more than one account and log a low level character in on my laptop to stand in the zone and act as a mentor target.
Which, no, is not an exploit either.