Doesn’t matter if it’s public or not, at least not in the US when it comes to COPA. If you’re allowing children under 13 to register with an email address, you have to get parental permission: https://www.ftc.gov/business-guidance/resources/childrens-online-privacy-protection-rule-six-step-compliance-plan-your-business

Edit: many states also have their own laws governing children joining social media platforms: https://wp.nyu.edu/compliance_enforcement/2025/03/10/childrens-online-privacy-recent-actions-by-the-states-and-the-ftc/

Afaik, there are laws and regulations that make it more difficult to collect personal information about minors including their email address. I imagine the admins understandably just don’t want to deal with that.

The free market works in mysterious ways.

This is the way!