I would phrase it "contracts" - which has the implication that if you violate the contract, you may be sued. Contracts are how just about everything is "enforced". I think that programmers tend to think that if something is required, we must have programmatic protections in place that ensure it actually happens. Most of the world does not work like that; they rely on contracts.

To handle noncompliers, they can get off of the various App stores with a simple DMCA request, no complicated lawsuits required, unless the developers appeal the take down, which probably involves a lot of "under penalty of perjury" lying.

It's unfortunate that this model really only works for large developers serving broad markets like gaming.

Whereas if you're an indie developer writing image processing code for example, it is going to be hard to find infringers and harder to strongarm them into coughing up licensing fees.

I expect this has the effect of keeping source code behind closed doors that would otherwise be public.

They don't necessarily need to do a lawsuit. They could just shut down your UE subscription until you file the relevant proof of earnings.