It happens because it can't be stopped. To register a new name, a user simply needs to register another email address. Most people around here have several email addresses (usually for perfectly legitimate purposes) so that's not going to be a barrier.
The only way that AB's server can identify users is by their IP address. Very few people have 'static' IP addresses. Nearly everyone has a 'dynamic' IP address. That means that they're given a different IP address every time they connect to the internet. So, if I wanted to create a new username here, I'd simply go to GMail or Gawab, to get a new email account. Then I'd disconnect from the internet and reconnect. I could then sign up again. AB's server would see a new email address and a different IP address, so there would be no reason for it to refuse registration.
Chris