Don't know why anyone was surprised
If you ever bother to read the license/terms of service for any of the decent game engines they're incredibly restrictive in all sorts of ways, and usually have some 'interesting' terms when you try to step outside a very narrow use case.
Add in the option some of them take to vary the terms from release to release and you're opening yourself up to all sorts of risks, assuming your concept is even compliant with their terms in the first place.
If they weren't so busy trying to squeeze the pennies and add restrictions they might all generate more money.
Biting the hand that feeds IT
