How does that work? If it's open source, I can get the source and run it for free, hell, I could even redistribute it for free. What's stopping me from doing this? I assumed it was that the non-free features were distributed under a proprietary license which comes with an invoice attached?
It's the same model than redhat, you buy a subscription, and the software checks if you have a valid subscription. Nothing is stopping you from modifying the source to remove that check, re-build and re-distribute for free (or for a fee) under another name. This allows community-driven projects like Centos or Fedora to exist. It's an ambitious business model, which doesn't prevent other to compete. The bet is companies that rely on the service will want to pay and will prefer getting professional services / hosting from the original maintainers.
I've pirated tons of stuff back in the days as a student, and know how to do all that even today. But now I've stopped pirating stuff.
It's partially because I've actually started earning money, but also because nowadays, it is incredibly easy to be able to PAY for stuff. And many companies actively integrate with local payment gateways and sell me stuff in my local currency too, which makes decision making much easier. It's actually become so easy that I straight-off decline when any friend of mine asks me to help them pirate softwares.