MAM used to be quite anti-VPN but I haven't used it in years, so no idea what their take is now. They tracked quotas and stuff through your IP and required you to be online on IRC. Great content and community, but a lot of hoops to jump through.
There used to be issues with tracking ratios when using a VPN. And since many private trackers require users to maintain a specific ratio, it meant that many private trackers effectively banned VPNs. Because if you were using a VPN, you’d be stuck at a 0.00 ratio and quickly get banned.
Usually they want only your IP while signing up to be able to see if they had already banned you and you try to evade it.
Most times there was the rule that once signed up, you can turn it back on for both torrenting and browsing.