Are you sure that's always the case?
School groups for my kids actually share that information as a way for parents to contact each other.
I'm not certain that we can say that any given email list is required to be private.
It's not always the case to be sure, but I'd argue that there's a large difference between making the intentional choice to share information and accidentally disclosing it to unintended parties.
"PII is defined as information: (i) that directly identifies an individual (e.g., name, address, social security number or other identifying number or code, telephone number, email address"