Recently we sent out our retention policy, which basically says that after you leave the University, you get 60 days of complete access, after which we prevent you from uploading new content and remove your source files (without touching your derivatives). Obviously we'd love to keep everything forever, but it's just not possible. The bulk of the content on Media Mill is stuff that doesn't necessarily gain additional value by having the originals stored, and the costs are substantial.
Anyways, there was some confusion from the email - we have no intention of actually billing folks for the storage space they use. We only mentioned it because we wanted you to know how generous CLA-OIT is.