The rules which govern us as pilots are a combination of the written FAR's, written FAA interpretaions, Admin Law precedent, and FAA enforcement philosophy.

Criminal and Civil law are also affected by precedent, but in those cases the precise wording of the written law is critical.

In FAR admin law, there seems to be far more allowance for interpretation of intent and practicality and this has been traditionally allowed by the ultimate arbiters (the NTSB).

In the case of DPE's, the FAA simply chooses to interpret a grey area in favor of their DPE's. This is probably because the FAA is obviously on good terms with the DPE community, and it also makes sense in that they want experienced, competent DPE's. If the FAA took a draconian approach to a DPE's commercial flying, the only people interested in being DPE's would be folks who never got beyond part 91 piston engines. It's probably best to have DPE's with a broad and diverse perspective on commercial aviation.