On the Anthem of the Seas this spring they required reservations for the bumper cars and they limited you to one or two reservations for the week. And you had to show the reservation on your phone when you arrived. You could walk up and half the time might get to go.
But I agree with what you were saying, given the demand, things like go-karts on NCL, iFly on Royal, etc., I don't think charging is unreasonable, like the arcades.