I only see the CentOS repos becoming visible but not enabled. This does not seem to affect the upgrade from 7.0 to 7.1 .
I've gone back and read the the bugzilla & other comments, and you are correct.
Base & updates are made available (but not enabled) in an upgrade. Whilst a clean install of 7.1 will have them enabled by default. I had enabled them manually myself.
Gordon recommends that they be enabled, but they weren't 'reset' on an upgrade to ensure that other packages etc. installed were not upset.