If you scroll down to the bottom of any page on the forum, you'll see a little note saying "Sponsored by Private Internet Access, a Bitcoin-accepting VPN." Since the forum is sponsored by a VPN, and so many of its users use Tor, VPNs, and other methods of anonymization to ensure their privacy, it would be rather surprising if the administrators decided to block VPN use. As hilariousandco mentioned, some VPNs and particularly Tor can result in a small fee when creating a new account (to prevent spam), but you can avoid this while preserving your anonymity by registering from a library or coffee shop if necessary.