It appears to be a bug with the game, It's locked for me despite my high level.
Coincidentally I have a Daily Objective requiring that I kill a player with a bounty.
Rockstar has disabled the Daily Objective relating to bounties on PS3 and 360 – this should prevent any players from being unable to complete their Daily Objectives while bounties are not available.
Again we apologize for this inconvenience.
Please refer to the “Cheat method affecting PC, PS3, and Xbox 360 systems in GTA Online” KB article for more information about this.
They also directed my attention to this article. Which states:-
We are aware of a new method where cheaters are manipulating the Rank and in-game funds of other players on the previous generation consoles (PS3 and Xbox 360) and PC in GTA Online. We are actively working on a fix for this issue, and may temporarily disable some minor game features while we work on a resolution. Please check back on this article for updates.