Should default settings be considered as business rules [...]
There are many posts explaning what business rules are exactly, but in the simplest definition I'd say business rules are the reason the application exists. Without business rules, there wouldn't be a need to create a program.
Business rules can have defaults, meaning if the user interface allows ...
A Default value is a piece of Business Logic.
Kevin takes an order to photocopy a sheet of paper
Kevin goes to the printer
Kevin sees that the piece of paper is A4
The customer did not specify the size of the photocopy
Kevin deduces from this lack of information, and the original being sized at A4 that the copy should be A4
Kevin selects the A4 Copy option
Your situation is indeed somewhat special, many config systems would only support overrides of defaults, but not complete "deletion". In many cases, you have a cascade of defaults, i.e. there are default values defined in the software which can be overridden in the system installation, which can in turn be overridden in individual use cases.