The definition of downloading was written into law before streaming became widespread; likely, when the former was used to prosecute someone for streaming copyrighted material, the defense pointed out the inadequacy of the existing definition. There's a distinction in law between downloading in the sense of saving something to your hard disk and just having it in RAM. Obviously, if I go to your website and see something on my screen, I have 'downloaded' in some fashion, but if just passing through memory was considered a download then running a program or opening a file would be considered equivalent to making a copy.
