It only kicks in after you make a certain revenue with the game (200k USD for Unity Personal and 1Million USD for Unity Pro). So if revenue is 0, it will never kick in.
The same way any company that has a contract dependent on the other side's revenue (or the tax services for that matter) does it. If you use Unity you have to report your revenue, and if you are caught lying their lawyers are going to shred you to pieces in court.
Also, you're liable for whatever amount their AI says you're liable for. And you're liable for whatever amount their install telemetry says you're liable for. Whichever is greater.