But it seems unnecessary given the existence of its laws. For example, CFAA makes unauthorized use of a computer system illegal regardless of whether the system communicates it. Perhaps these messages originated prior to the CFAA, in which case perhaps they were necessary back then, and nobody has dared to remove them.

This is unlike trespassing in the US, however, which does require informing the person (written in a conspicuous location, or direct verbal conversation) they are unwelcome on whatever land they began accessing, and allowing them to promptly retreat, before any violation is committed. Access is generally permitted (e.g., to allow for unsolicited deliveries) prior to such communication.