Vehicles with a manual transmission similarly start engine braking the moment you lift your foot off the accelerator but I never attributed any sense of motion sickness to that, though the braking force likely is lesser.
The way I've always understood this is that since passengers usually sit in the rear they're looking out the side windows and that scenery flying by so fast is what causes that motion sickness and it doesn't happen to the driver or front-seat passenger so much because they're looking forward through the windshield.