I there are several factors that should make the NBA championship more important to a player than the Olympics.
1) The effort required to win an NBA championship is much more that the Olympics. A full training camp, pre-season, 82 game regular season, and four playoff series is required. The Olympics is like a summer vacation compared to that. The NBA is the primary focus of a professional basketball player. The Olympics are always secondary.
2) Like it or not, winning a title is the standard by which careers are judged. Charles Barkley, Patrick Ewing and Karl Malone will always be remembered for not winning a title, despite all being Olympic gold medalists. If LeBron feels the gold medal is more important right now, I can understand that. But history will judge his career on whether he wins a title regardless of how he feels. Does anyone dispute that?
3) A player contributes more value to a championship team compared to the Olympic team. Restated, each player on the Olympic team plays less of a role in the team's success compared to their professional team. I argue that you could remove any player from this year's team and the United States still would have (or should have) won. The same can not be said for the NBA championship. Remove any of the big 3 and the Celtics don't win a title. You could even argue that we don't win a title without Rondo, Perk, PJ Brown, and Posey. If you removed team USA's best player, LeBron James, there are plenty of stars to fill the void and the US would still be the favorites.