I guess this is a holdover from the old days when users were able to easily undelete their own posts, but now that you can no longer undelete any post deleted by a moderator, I think the automatic lock is no longer necessary.
Yeah, strictly-speaking it's not necessary anymore.
It does send a rather clear message though, eh? I mean, you can't even edit the post. Mod-deletion at least gives you the option of improving, flagging, and seeing it undeleted, but community flag-deletion salts the earth.