Oh, I'm sure the NBA would love that, too. Unpaid internships are great for team owners. The downside right though is that, say there are ~10 players a team targets as potentials, but need some seasoning. Is a team better off letting them blow up in college and being drafted by another team, or taking a player maybe a year too earlier and developing them under their control. Baseball used to have a way around this with draft-and-follow, but they did away with that.
Edited to add: 90% of the guys in the D-League are there so that the odd handful of real prospects can play/practice against decent competition.