So how is the GM restricted? Apparently I didn’t catch this.
I’m not sure what you’re asking. Can you perhaps expand on your question? Everyone is constrained by the scene budget and the GM has to spread things among his FONs.
If the GM goes hard on his players, what prevents him from killing them all?
So, the thing about BE is that the GM has to do everything except for set consequences for failed rolls through the GM FoNs. You have a lot of leeway when it comes to saying “yes” to things, but you don’t have the complete freedom to just throw things at the PCs. If they want to sneak into the secret medical facility of your villainous Vaylen doctor, then any security that exists in there has to come out of that NPC’s resources, and establishing that stuff will cost you building scenes. Henchmen will come out of your Circles pool, and eventually, you’ll have to start rolling and risking failure for that stuff. The GM has to play by most of the same rules as the rest of the players, so that limits the ability to just throw stuff at the players and steamroll them.
Where is this in the book?
page 620 “GM Authority” and on through the rest of that chapter.