I'm with you. Disable a save button makes sense if the data to be saved is not valid e.g mandatory field missing, wrong value etc. In this case the invalid / missing fields should be clearly marked with instructions how to fix it. This way it is clear to the user why the save button is disabled.
to offer yet another perspective,
I would advise to not block any user actions that do no possible harm to the user.
For all situations when clicking save would cause no harm, leave it enabled. Disabling potentially makes the user think: "I want to click this button, why can't I click this button? What should I do to enable it?" If there is no harm, all ...