I'm in favor of auto-locking, but I'll play devil's advocate for a bit and list the potential cons I can think of:
1. Would make it easier to spam lwc-locked chests, producing grief that needs an admin to remove lwc. If our lwc database gets pruned regularly, we can disregard this point.
2. Making quick shared or communal chests is a little more hassle: have to /cremove then /cpublic. This might confuse friends playing on pve for the first time (although the more prevalent cognate problem of ignorance of /cprivate is more pervasive and pressing).