Unfortunately there isn’t an easy fix for that. However, I agree that it should be an option, so I’ve opened a feature request GitHub Issue and will work on creating an option for this in the next release. I recommend following the GitHub Issue to stay updated.
This feature has been added to the latest version of the theme (v1.4):
Enhancement: The Primary Navigation menu can now be shown on Single Post pages. There is a new “Show Nav Menu on Single Posts” option in Appearance -> Customize -> General Options. See Issue #82.
See the announcement post here: http://independentpublisher.me/2014/independent-publisher-v1-4-1/