This was intentional. The NBA has spent some time trying to figure out what exactly it wants the "G League"/NBDL to be. The way I see it, there is really no downside to the league investing more money into the G League. If they could get it to a point where the salaries were respectable, maybe it could offer a solution to the one and done model? I know NBA folks were at least partially against the straight to the NBA out of high school rule because it meant larger risk for NBA teams. High school prospects are the definition of hit and miss. For every Lebron or Garnett there are 10 Kwame Brown's.
If the G League pay was respectable, and the age rule was removed, teams could simply draft players and stash them in the G League where they would have time to develop and improve prior to playing with the big boys. This is already happening to some degree. Just my .02