[spydum]

I thought they did not have the "reason" for the arrest -- only the warrant.

The arrest may have nothing to do with accessing the Public FTP, and entirely to do with the research he was doing on the FTP service itself. If he was attempting to exploit the FTP service hosted by someone else (something or other aboubt database credentials was mentioned), he would absolutely be in violation of CFAA. You do that sort of research on your OWN system.

First rule of security testing: make sure you have permission.