" Domestic airplanes are basically designed to keep people seated, settled, and sedated for the duration of flight. If they could, they would install catheters in the seats and strap the people down from takeoff until landing. International flights are a little different, due to the part where sometimes people’s veins explode if they sit still in a pressurized cabin for too long. (This may be a small exaggeration—emphasis on “small,” not “exaggeration.” Deep vein thrombosis is the silent killer of the long-haul flight.) To combat this, international carriers often encourage people to get up, move around, and keep their blood circulating normally. Sure, it means the aisles get a little crowded from time to time, and it makes the TSA nervous, but better that than a bunch of dead passengers. "
― Seanan McGuire , Pocket Apocalypse (InCryptid, #4)