I work in a large US software company which deals with UGC (user generated content).

Illegal content is serious shit. We have systems which MOVE the illegal data when we find it to non-production machines. We are also required to notify the authorities. The authorities come and potentially REMOVE THE HARD DRIVES with the illegal data. If we don't move the data off the production machines, they have the authority to shut them down and/or seize them.

Not removing illegal content that you are aware of is very serious.