Expected import behavior when setting custom field to view only
It seems a probable bug. 
I ran an import of a file containing same field as my custom field and the field was updated by the information mentioned in csv and did not retain its default value. So, Yes, the issue was replicated. 
The value of custom field is only retained when we create a contact through CiviCRM and not importing, so it looks to be an inconsistency in the behaviour while importing. An issue will be opened for the same in issue tracker of CiviCRM.  Thanks for reporting!