Why not? What's the reasoning behind this? (They can only lock conversations they have started, they can't lock other conversations and it doesn't prevent moderators from starting a conversation with them).
Members should not be able to control the flow of discourse with staff. I've now had multiple times where a member starts a conversation with a mod/admin that requires a reply but the member has locked the conversation. It's very frustrating and a waste of time to start a new conversation with that member.
I have to agree with dethfire. I just started using XF again after a long break and didn't know this was possible, it shouldn't be. A troublesome member could easily use this ability to intentionally irritate staff members.
This has been a feature of Conversation Essentials for a long time though it's implemented in a different way. I know $45 is a bit expensive, if you want just this feature, so I went ahead and made a new add-on with just this feature this morning since I woke up a little early for work. https://xenforo.com/community/resources/conversation-lock-bypass.5102/