If you add security level as a field to Workflow transitions, then the user is able to change the security level field when transitioning an issue.
For subtasks, you should not be able to set this value explicitly, as the subtask should always take on the security level of its parent.

For a subtask workflow transition, the user is presented with an editable drop down for the security level.
The rest of the code correctly ignores this new value and leaves the parent's security level, but it would be better to
not show the drop down at all.
eg show a read-only message.
See for instance the behaviour in v4.0 when you try to Bulk Edit a subtask only.