I'd guess that this is to ensure one abusive user doesn't get every other user blocked from a large service (say, Google) for botting over the VPN and constantly rotating IPs.

It's a practical measure, but definitely has a privacy cost though.

It's possible that contributes, but to be honest most VPN users are split "privacy seeking" and "abusive". Though I grant you paid users are probably slightly more circumspect than users of Tor, etc.

It seems more likely this is just about load-balancing use against their available nodes.