It is rare that sites need a ton of storage unless they are hard core photography sites, or based on file sharing. Lots of services like YouTube, SoundCloud, Flickr, etc let people host audio, video, and big pictures for nothing.
My point is that embedding remote content is way more common than uploading.
Beyond that, we do believe in automagically mirroring remotely linked images so topics do not break later when someone removes a remote image on some remote server. That is why remote mirroring is on by default.