I think the NBA has 2 way contracts that allow NBA teams to send you to the D League without losing your rights.
Just googled to confirm. Apparently they call it the G League now, but I was right. The 2 way contracts restrict those players to 45 days max in the NBA. These contracts simulate the MiLB/MLB construct except for the limit on the amount of time a player can spend in the top league. The other exception is that most G League players can be picked up at any time by an NBA team for as many as two 10 day contracts and then they could sign a full contract for the remainder of the season if they perform well.
The upside to the 2 way contracts is that those players make about 3 times more than other G League players so there's a little more security there.
The downside is you lose the ability to move to any team in the league and your potential earnings are limited. A 2 way player can only max out at 45 days of NBA pay with the rest at the G League rate. A G League player who isn't on a 2 way contract could theoretically get picked up by any one of 30 teams early in the season and get rookie NBA pay for more than 180 days.
I can see why KDub would turn that down if he believes in himself.