Finding interesting tactics to get an upperhand in the game (protip: thats the defintion of exploit), is not a recommended hobby when wanting to keep your account.
I'll try this one more time. Some people would consider the use of a 70 to instance boost a lowbie, just such an "interesting tactic". See the similarity? Forget the actual tactics. I realize that thus far Blizzard hasn't taken that stance or they'd have "hot fixed" it. That doesn't change the fact that with their tolerance of instance boosting (or boosting at all) they are at a minimum passively endorsing the tactic. Therefore, those that used Zolo (I never had the chance myself) very easily could have seen it as nothing more than following precedence. Either way is sort of a "cheesey" approach to leveling in that the lowbies do NOTHING and gain levels. Sound like an exploit or an intended game mechanic?

So, if someone finds an event, instance, mob-set that generates a higher rate of exp than a comparable instance, that's automatically an exploit? That's some stretch considering the policies that are in place.