Why is the Departure leg of the traffic pattern being mistaught as Upwind?
Your Question:


  Why is the Departure leg of the traffic pattern being mistaught as Upwind?


Probably because it is quite common to use the term "Upwind" when flying the "Departure Leg" because when doing so, you are flying "upwind."  If you look at the Pilot/Controller Glossary  in both the AIM and ATC Handbook (same P/C glossary), the term "Departure" leg is not even mentioned.  It only shows in the body of the AIM.

Here is what the Pilot/Controller Glossary defines for a "Traffic Pattern:"

(emphasis mine)


  TRAFFIC  PATTERN
  −
    The  traffic  flow  that  is
  prescribed 
  for aircraft landing at, taxiing on, or taking
  off  from  an  airport.  The  components  of  a  typical
  traffic  pattern  are  upwind  leg,  crosswind  leg,
  downwind leg, base leg, and final approach.


I'm not suggesting that the segment in the pattern you are referring to is not the "Departure leg," just that it is quite common to refer to this segment of the pattern while you are flying it as "upwind" (without the use of the word "leg").  The actual "Upwind leg" (as I'm certain you know) in a traffic pattern is flown parallel to the runway on the opposite side of the "Downwind leg".