With anonymous I think matt_f was referring to the users, not the creator or manager of the service, the issue being that an anonymous user could use the service to transmit illegal/immoral/... information (stolen CC info or identity data, CP, trade secrets, government secrets, and so forth) using the service.
The assumption that makes this a problem is that the service makes such users (at least slightly) less easy to trace, and/or the person(s) running the service might in some jurisdictions be seen as responsible for enabling the illegal activity.
The assumption that makes this a problem is that the service makes such users (at least slightly) less easy to trace, and/or the person(s) running the service might in some jurisdictions be seen as responsible for enabling the illegal activity.