Bukkit restricts command execution of signs to test if the sender
has permission to run the specified command. This breaks vanilla
maps that use signs to intentionally run as elevated permission.
Bukkit provides an unrestricted advancements setting, so this setting
compliments that one and allows for unrestricted signs.
We still filter sign update packets to strip out commands at edit phase,
however there is no sanity in ever expecting creative mode to not be
able to create signs with any command.
Creative servers should absolutely never enable this.
Non creative servers, enable at own risk!!!
Aikar