fsync may only be run against the admin database

I'm experimenting the fsync command before launching it from the backup script through mongo --eval before the backup process.

My questions would be :

(1) Why is mongo not allowing to run the fsync command on the non-admin database ?

(2) Will running fsync command in the admin database will fsync for all databases for the active mongod ?

Example of running fsync for a non-admin database :

mongo <myDb>

rs0:PRIMARY> db.runCommand({ fsync: 1, async: false, lock: false })

{

        "ok" : 0,

        "errmsg" : "fsync may only be run against the admin database.",

        "code" : 13

}

Thank you !