Originally Posted by
GW McLintock
This is an FAA requirement and while I don't know the specific citation it is enforced in air crew training programs.
It is not an FAA requirement. See below what the FAA says:
It is up to the discretion of the airlines whether having a CRS in a non-window seat would block the escape path in an emergency (and as CARES harnesses don't impede anything, an airline not taking a "bluntness of a hammer" approach per SWCPHX might distinguish between the two in its policymaking).
Case in point, Southwest permits in window or middle (but not aisle):
https://www.southwest.com/help/flyin...g-with-infants