When an NBA team wins the championship, they are called world champions. How did that come about?
I think this is because NBA is probaly the most important national basketball championship in the world.
Even Dream Team Olympic performances in world vision helped the thought that the NBA tournament was at a higher level than any other world championships.
The Guardian has analyzed this issue and emphasized the non-validity of this appellative.
However, any of this elements does not justify the "World Champions" title.
Like it or not, American sports has a history of calling the winner of each of its major professional sports each year "World Champions." This tradition goes back to the 1884 World Series. Besides the NBA and Major League Baseball, the NFL also calls their Super Bowl winning team "World Champions." It is a little bit of a joke, but also signifies that we American sports fans think of our sports as the best in the world.
It's a simple premise. The best players in the entire world (including the best international players) play in the NBA. Therefore, champions = "world champions", aka winners at the highest level of basketball competition.