If they didn't prevent selling derivative game engines, someone could fork it and add a valuable feature that was only available in the paid fork. This could split the community.
(it's not)
If they do, look out.
Unlike Defold, both Godot and O3DE are actually free.
That number is vastly smaller.
If Godot games were making anywhere near the revenue that Untity games do, I’m willing to bet they’d there would be an Amazon fork.
(and it's still bigger for Godot)
It also doesn’t matter what the user base is today. It matters what it might be tomorrow. It’s not easy to change your license. If you wait until Amazon is coming for you, it’s too late to do anything to stop it.