Yeah good point. I know people that get mothion sickness from FPS vehicle sections. Flips would just be dizzying.
In an interview with SD's lead writer, I heard him talking about this very thing. About how the camera swaying and movement that many first person parkour games have had, and how it can cause motion sickness and some. Basically, at the very start, Aubrey Hesselgren strapped a camera to his head and did some parkour (as well as dodging punches from his dad) to show that it doesn't really give much information. Your brain, eyes, neck, and body all automatically compensate for movement making things very smooth overall. In real life, you are aware of where the different parts of your body are, without looking at them. So SD had to be a bit creative in how to make the parkour intuitive when all the information you get is visual. Bobbing the camera around doesn't do much, and prevents aiming (something they wanted you to be able to do at all times).
But basically, the screen stays fairly level even during parkour, to the point of allowing you to fire from the hip.