|laziness, impatience, and hubris|
The attackers, if they get control of the server, could then only decrypt the files the users download by sniffing passphrases.
If the bad guys get access to the server, tweaking the download script to forward the filenames and associated decryption key to some dead-drop would be trivial.
Done with a little care, the server/site owner need never know the script had be tweaked unless they happened across the two or three lines of code that would need to be added to the source. Done with a little more care -- secreted into the source of a standard module called by the decryption script -- even that risk of discovery can be mitigated.
It still requires the user to supply an encryption key prior to the file's creation; and to have a separately encrypted copy of a given file for each user that will download it.
With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.