I believe the NBA technically doesn't have an injury list so the Yahoo Leagues just followed. I believe the NBA changed its rules a couple of years ago because teams were manipulating their lineups by placing uninjured players on the IL.

Here is the official answer from Yahoo:

Is there an injured list spot on my roster?

No, there is no IL spot in Yahoo! Sports Fantasy Basketball rosters.

The NBA no longer uses an Injured List (IL). Instead, real teams designate certain players as "inactive" just prior to game time. This designation means that a player is on the team's roster but not eligible to play in the game for which they're listed.

NBA teams are now required to carry at least 13 players, but no more than 15. Twelve of these players must be active, meaning that from one to three players must be inactive for any given game. The inactive list can include, but is not limited to, injured players.

As a result of these official changes, the IL option has been removed from the fantasy game. If you want to replace an injured player in your starting lineup, you must place him on your bench or drop him to waivers. Custom League commissioners can choose to add another bench spot to rosters when they configure their league if they deem it appropriate to offer additional flexibility to league managers.