Hi,
Use get conf | in ethx/y to display all the references to the port. It may be used as the source port for syslog, tunnel interface, ntp etc, etc. It may be a parent interface for a tunnel interface or something like that. With other words, there are many reasons why a port is protected from editing. This is a reasonable limitation. Certain changes may destroy the configuration integrity if they are applied to an object which is linked to other objects as a parent one.