No idea for their specific case, but usually because VPNs makes it easier to circumvent bans as far as I am aware that is the main reason.
Not like it helps much tough as most people have dynamic IPs anyway… Any IP you have a suspicious of being of a banned person could just be of another ISP customer… Or the banned guy could have now another ip… so a user could circumvent a ban without a VPN anyway. That’s I guess why most Trackers stopped caring about the VPN.
Another reason could be to avoid crawling the content… That again would be related with banning the user anyway and nowadays with Jackett and company that is either expected or there is an API for it.