Certainly, this reeks of exploit. I think it's natural that people do this with the ridiculous xp curve imposed on them though. Not being able to retry failed leves is a non-starter option, every single leve I've ever failed has been due to a game crash or d/c, and it's unbelievably frustrating that they even fail in the first place, let alone that you couldn't retry them before.

Considering that people will, in general, want to maximize the amount of leves they do, and there's a finite (too small) amount of leves available per level as it is, they should probably normalize the difficulty and sp gain of the leves to remove the motivation to do this. It'd be better to have all leves give roughly the same amount of reward for time investment then having people pick and choose leves in far off places and leave some leves completely untouched. Not a totally ideal solution, but I believe it's better than the alternative.