Are all topics locked by admins unable to be reopened by the user? I guess I remember having a topic that was locked by the administrator and I reopened it but I'm not sure. 
I believe the answer is yes. Although so far none of the topics I created have been locked by moderators but users will lose access to unlock topics if moderators have locked them. If the topic owner can reopen topics locked by moderators due to a lot of spam, then the moderator's actions at that time are useless. I believe the moderator's action when locking the topic is a well-considered final decision. Maybe for a different case the OP can make a request to the moderator to reopen the topic if something needs updating, but in most cases the locked topic is a topic full of spam so it's impossible to reopen.