It’s uncommon for ‘public use’ ethernet ports to exist, unless they are clearly labeled. The ethernet ports might grant access to the internal network, which, is easy to accidentally do. A non-profit library with a limited budget might overlook all the extra protections on open ports (enable/disable ports as needed, use 802.11x port-based authentication, internal SSL, etc), that would be necessary to secure it. Or, even better; that RJ45 port might be wired up to an old PBX, and you may have fried their telephone system, or your own hardware.