It seems that Amazon and Apple don't need to be worried, but artists should be scared.
Only true if Spotify completely replaces iTunes downloads, which doesn't look to be anywhere close to happening at this point in time.
You could say Spotify is a nice way for artists to supplement their income. Great for introducing your work, or getting cash from people who otherwise wouldn't buy any of your material. If I were a musician, I wouldn't want to rely solely on Spotify to make my living, but I wouldn't exactly bash it either.