An exploit would suggest they are doing something the game itself shouldn't allow such as being able to beat Garuda with a bunch of BLM and preventing Garuda from usig a 2hr. That's an exploit because you broke the system by making it so that Garuda is unable to use her move because of how much dmg she has taken.

However what you are saying is people are using a leve charge to start a leve, then they just abandon it and they recharge all of their abilities. Now there's nothing wrong with what's going on they choose to use a leve charge so they can get their 15min back if that's how they choose to make use of it then that's their choice. It's not like they are doing it for free or anything that do have to use up a charge if they want this to work.

If it was something like doing a leve in Dragonhead and then canceling it causes the 15min wait time for Garuda to dissapear then that would be an exploit because something in the system is causing the game to act in a way it shouldn't.