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.
I would say that fewer people will use tor because they will generally need to pay a small fee to register which people generally do not want to do. Plus the recent number of exit node attacks have made accessing a non-onion site via tor subject to a potential MITM attack. (I do think the forum should have an onion address that people can connect to tor through and generally not be subject to such attacks). I believe that people who sign up via PIA will generally not need to pay any kind of fee.
