If authorities ask you/demand with a court order "who uploaded this file" you can hand out his ip. I also believe( haven't checked his one) you must retain a copy of all files for a short period of time even if they inaccessible to end users.
Privacy yes, but legal? If you operate a website/ service that may be used for illegal purposes then I think you need to be able to track down illegal activities.
The US actually does not have any kind of legally mandated data retention for internet services. If you do not log the data, you cannot be compelled to turn it over.
Why?