Why should a lectern be lockable
You might say that only "normal" inventories can be locked but beacons can also be locked and beacons are not an inventory. You would want to lock your lectern if you only want some people to have access to it. Also, you can control Redstone systems with lecterns so locking lecterns would be useful for maps that you can only access them. If you a key item. Or if you just want to secure Redstone systems.
When you update the nbt data of the lectern with:
You can still open the block as normal without needing an item renamed to lock.
How to reproduce:
1. Place down a lectern
2. Run /data merge block <coords>
at the location of the lectern
The lectern is not locked and I can read what is inside.