1. The NBA has a specific rule requiring players to stand during the National Anthem.
2. NBA players are very image-conscious. I think the likes of LeBron James--who has spent a large fraction of the money he's earned as a player and from very large endorsement contracts to help the downtrodden in the Akron and Cleveland areas--may not want to be associated with taking an excessively polarizing stand--he has too much to lose financially and PR-wise.