I think it is not intended behaviour that, when for a tag field in a database, the option "allow tag creation" is active, then now as a user you cannot create new tags when the database is set on "Lock structure".
You do not alter the structure of the database when a user creates a new tag, and through a setting, this is permitted. Moreover, it is the intention of the database manager to have users create new tags.
The "lock structure" option certainly is a very valid option, and I should be able to lock the structure AND allow users to create new tags, if I wish so.