Tell that to the valid copyright holders hosting their content on MegaUpload. Regardless of whether the system is used for legal or infringing purposes, the reality is that distribution companies have the ability to take down both when they claim some are using it to infringe. The concern for copyright holders is not the encryption per say, but the ability for big business to NOT be able to take down their legal content under the guise of a moral cause.
As soon as the sharing group got big enough to notice, then there could be agents in the group reporting infringement.