NBDL players are contractually obligated to the league itself, not the club for which they play for. NBA players who have been relegated to the D-League by their major league teams remain obligated to their NBA contracts of which supposedly each NBDL roster is required to have at least two contracted NBA players (in the case of Reno Big Horns I guess would be two total from Kings/Warriors). So players not signed to an NBA contract are free to sign with anyone - although perhaps their tiny NBDL season contract might have to be "bought out."
Of note: It's been reported, average NBA D-League salary is $12,000 to $24,000 a season. While NBA players are paid $100 a day for expenses when they are traveling to and from road games, NBA D-League players are paid $30.00. Average NBA player salary is $5.8 million which is significantly higher than average salary in other major pro team sports. It's claimed that over past two years average NBA salary has declined about 10% but new CBA proposed by the league would cut it further - the question is how much?