Its not the EC's fault but sometimes these "codes" dont have ANY LOGIC in them.
If people understand what the code is and how to use it then it's very logical ( in most cases). The code is just a set of minimum standards used for new construction. It's not a design manual.
The code makes allowances for all sorts of things to include special permission granted from the AHJ.
They could probably have gotten special permission from the AHJ with just a phone call if adding that 20 Amp circuit was going to be a great financial burden on the homeowner. If they really were going to have to tear half the house apart. But if it's only a matter of fishing the cable to the basement and over to the panel that's a horse of a different color.
The difference between getting special permisssion granted by the AHJ and the inspector just turning a blind eye is that one method is legal and the other is not. One method protects the contractor. To operate any business it's important to know how to navigate through the laws that govern said business.
When it was first noticed that this was a 15 Amp circuit that was the time to call and ask for permission to leave it as is. That way if the head electrical inspector needs to run it by the building code official there is plenty of time. If permission is granted then the first inspector would not have red taged the job and the problem would have been solved.
That's why we carry these little cell phones around, it's to communicate. Ten minutes on the phone is not that big of a deal.