As Cerbrus said, it's purely a legal issue. We're not allowed to collect and store any personally-identifying information about minors under the age of 13 without explicit parental consent.

If it's discovered that someone is under 13, we:

Send them a courtesy email letting them know that we have to remove their account, and why. We also strongly encourage them to come back once they turn 13, and place heavy emphasis on the fact that they didn't do anything wrong. We encourage them to keep going with what they're doing. This is not a canned email, a community manager writes something every time.

Sometimes get a response from the parent, and we instruct the parent on how they can have an account, and post questions for their child as needed. But, it's the parent's information we're collecting.

While I really wish we had the resources to set up a department that just kept track of parental consent and users under 13, when they turn 13 and everything else - we simply don't. That's why we have to purge those accounts when we find them.

We can't really do much more than we do for anonymous use of the site beyond simply reading it. The number of cases of cookie-only accounts on Stack Overflow inundated our support department so strongly that we had to require registration in order to ask a question - that many people were losing the cookie and becoming disenfranchised from their accounts. This meant they couldn't comment, so they wrote answers. Or they'd suggest edits to their own questions or answers, it was a mess. We need something tying you to an account to offer any hopes of recovering it.

I wish we didn't have to do it, but being the parent of two kids under 13 and a former programmer that's well aware of the scum-baggery that junk sites delight in delving into .. I appreciate what the law was trying to accomplish. It doesn't stop kids from being tricked into giving this information to sleazy outfits that abuse it, but it makes what those sleazy outfits are doing a crime - something that comes with consequences, and that's something I appreciate.

Update:

It doesn't matter where our servers are located. What matters is where we're incorporated. We're a US corporation, and we have to follow the law.