CNC machines are no longer air-gapped. They sit on OT networks. An infected transfer tool can carry ransomware or keyloggers. Imagine a keylogger capturing your FANUC parameters, or ransomware locking the control panel of a $200,000 machining center. Unverified downloads are a primary vector for industrial sabotage.
Once you have secured a verified download of the Fanuc PTT, implementation should be handled with care: fanuc program transfer tool verified download