I doubt it's an oversight. I was about to upgrade last time they had a similar offer, and I called them when the disount didn't work. (It's not immediately obvious on the website as you don't get an error message if you apply the discount code, instead nothing happens).
The guy pretty much said the discount was intended for full product purchases.
Would love it if they changed it though.
--------------
http://www.jonbagge.net
Jon Bagge - Editor - London, UK
Avid - FCP - After Effects