This might be Pega OOTB design for handling locking (disabling locking option in child case - Optimistic Locking) in later version of Pega.
Default Locking = Only 1 user can access the case.
Optimistic Locking = Multiple users can access the case.
Basically how locking will work is, parent case will have its own locking and child case will inherit lock from parent case. Based on business scenario if we need to change the locking of child case then we will explicitly change the locking mechanism in child case.
In later version of Pega if parent case is having default locking, then Pega providing option to change the locking of child case based on business need.
If parent case is having optimistic locking, then Pega not providing option to change the locking of child case. In this scenario both parent and child case will have optimistic locking.