That is because they both have subforums. If they were not locked, you could post into them, where you are only supposed to post in the subforums. Usually, it does not really matter, though it does look somewhat funny when a boardwide announcement is posted.
On a side note, on a previous incarnation of the boards, the Locked symbols had been replaced by the normal forum symbol. i don't know if it is possible to duplicate this here, though.