it should reflect the marginal cost to them of providing you with the service
You are saying that because you know full well that the marginal cost to them is effectively zero, and you would therefore get it for nothing.
In business, you don't offer your products for sale at their marginal cost, for the very obvious reason that if you did you would never make any profit.
If you use their game engine to make a game that sells millions and earns you a fortune, it seems entirely reasonable that they get a proportion of that fortune. You are not being forced to use their particular game engine, after all. There are others out there, or you could roll your own.