You are underinformed.
Appraisal standards (including but not limited to USPAP) don't create the fundamentals of appraising; they reflect them. HBU isn't some optional add-on to the appraisal process; it's a crucial and integral part of the process for an appraisal being performed under this definition of MV. Assignment conditions - including lender guidelines and policies - can always ADD to the fundamental process but they can never UNDERMINE that process to the point that the result doesn't reflect the value being asserted. MV itself is based on the actions of the market participants, not the preferences of the lenders for expediency.
Besides, with only a couple exceptions, each of the requirements of USPAP reflect what a conscientious appraiser would choose to do in an assignment if they knew what they were doing and why they were doing it. The fact that there are a lot of conscientious appraisers who have little or no experience (i.e., competence) with performing an HBU analysis doesn't change any of the reasons why it's required in the assignments being performed under this definition of MV. The fact that it's possible to make the assumption about a property's HBU instead of actively considering it doesn't change any of the reasons why it's required in the assignments being performed under this definition of MV.
If the MV of the extra parcel is higher when sold off separately than if used as additional lot area for the improved parcel it becomes an untruth to say otherwise. Whether that untruth occurs due to a violation of the COMPETENCY RULE (the appraiser didn't understand the HBU issue) or a violation of the ETHICS RULE (the appraiser knew better but did it anyway) it's still unprofessional conduct. No edict Fannie can issue can alter that untruth. They can ignore that untruth on their end in the name of expediency if they want, but they can't instruct the appraiser to ignore it.