As outlined in Case # 2783413
We were unable to share an link to an EPUB which had watermarking embedded in the file.
Workarounds included zipping up the EPUB first.
It would be wonderful to get full EPUB support where you can view the EPUB (ie see the cover image, and even the contents of the EPUB book and allow anyone with the link to download the file.

As was discovered in the case mentioned above, it turns out some EPUBs can at least be downloaded if they didn't contain watermarking. Publishers put in watermarking so removing these is not possible. Box should allow anyone with the link as your own U.I. suggests can download the file.