The biggest reason we don't get hit with massive DMCA filings is because the fanfiction ecosystem is a publicity cash cow cult for the corporations that own the IPs. The second biggest is that it's a small money pot in corporate power terms. But once big enough and lucrative enough, or someone at Disney has a bad day, the corpos can fire down on multiple targets for damages. SH can be sued for damages if some corporation thinks their pot is being siphoned or their reputation being damaged by smut Jedi tentacle warriors having orgies.
My biggest fear in the fan fiction debate is more of a long term vision. I'm afraid corporate intelectual property concepts are going to bleed into concepts, terms, ideas, and genres. For example, not being able to have light sabers in original space opera, not being able to use certain alien races and settings, the shape of space ships, not being able to use creature concepts because corpos mass produce a trademark library of them, etc. Thus further limiting what original writers can produce via cultural ownership and cross referencing. But that goes far beyond the debate about what SH needs to do protect itself from legal liability now.
One of the reason I don't like fanfiction is that it feeds the beast in the paragraph above, because anything new or original authors produce in IP worlds becomes potential IP property for the corporate properties as well.
