They do not actually inherit from the base RightsBuilder.  They HydraRightsBuilder and BDRRightsBuilder interact with the base RightsBuilder through composition.

It may be a naming thing.  Some other name for the base or the Specific Builders might be better.

That said, a common class could be extracted that would contain the common logic for is_person, but I also thought that it was particular to each implementation and is more of a coincidence that they both care about whether someone "is a person".

Very open to debate, just sharing reasoning.