This understandable requirement, probably a result of US legislation (and perhaps legislation in other jurisdictions?), is at odds with the Raspberry Pi's stated focus on children (hence the "considered harmful" jab).
In the US, the Children's Online Privacy Protection Act of 1998 makes it very complex to allow children under 13 to participate in social media, because there are so many rules about getting information from children without parental consent. That's why sites like Facebook require a member to be at least 13, though lots of kids violate this by lying about their age.
For a younger child, the parent and child can participate in the forum together, so the parent has access to any communication or advertising directed at the child.
About Chris Tyler
I am a Christian, college professor, computer programmer, system administrator, author, and consultant. I am also an Industrial Research Chair with a focus on Linux on 64-bit ARM enterprise systems.