Consider why you carry approach additives -
(a) steady wind is expected to decrease within the latter stages of the approach and landing. Thus, we expect to see a shear - in the case of a tailwind, a reducing tailwind. There would be no sensible reason to carry an additive in this situation.
(b) a gust pattern, superimposed on the steady wind is unpredictable, so it is desirable to provide for gust events.
There was some discussion on this subject in an
old Tech Log thread. More recent threads have identified the Boeing practice as having been introduced during the early 707 days.