Cancelling uploads can have lots of reasons.
I have cancelled uploads on a few occasions myself, because I have my PC in my bedroom, and it makes far too much noise for me to be able to have it on all night.

So if I'm downloading a small file late in the evening, or have downloaded a file for some time and it's about to finish at such a late hour, and someone is beginning an upload from me, which will take him like 6 hours to complete on his 56K modem, I just have to cancel him.