VPN providers often don't run their own networks, but instead run on other ISP networks.

Those underlying ISPs have data that can be used to correlate (with a scary amount of accuracy in a remarkably short space of time) your flow to the VPN server against a flow from the VPN server to a remote host.

Without exaggerating, lives have been saved through this technique. Once the technology exists for one purpose, it can be repurposed very easily.