I'm certainly not the most up to date on all the RIAA's done, etc. however
It seems to me that it's the DUPLICATION that is an infringement of copyright and not simply making something available.
Putting my MP3s in my shared music folder so I can listen to them at work is not necessarily a copyright infringement. When my neighbor copies them, however, so he can listen to them for his enjoyment and not pay for the license to do that, well then, THAT is the infringement.
Just like photocopier days ... if I forgot my musical score on the photocopier after legally making copies for my music students, and someone else makes a copy, THEY are the ones that have infringed, not me. I suppose it could be argued that I was negligent in protecting their IP, but that wasn't really part of the agreement when I purchased the score for classroom use (and presumably paid for that level of use).
I'm sure it's all been hashed about before, but what am I missing?