B+ was considered a bit below par on the GUI side (so some said)
It didn't live up to BRL's hype, coupled with the fact that the windows MaxGUI files are still using most of the code from B+ anyway. Since a lot of people were disgruntled and only the Mac and Linux code was really new... a discount to the previous Windows users makes sense. I still think giving it away for free was too much... but that's BRL for ya.