It is not unusual, particularly on long summer road trips, to see front seat passengers with their feet on the dashboard.
Passenger side front airbags are standard in any vehicle built after 1998. In the event of a collision exceeding a certain threshold (typically 25-40 KM/h), they deploy from the dashboard at exceptionally high speeds. The speed of the airbag is necessary to be effective in preventing a passenger from striking their head on the dashboard.
While an airbag is effective in the case where a passenger is seated appropriately, in the case where a passenger has their feet on a dashboard even a minor collision can be disastrous. It is not uncommon for a passenger sitting with their feet on the dashboard to suffer severe foot fractures, facial fractures or traumatic brain injuries due to the speed of the deployment of the airbag.
In the event that a passenger sues an at fault driver for compensation arising from these injuries, the passenger could be held partially responsible for their injuries for failing to take care for their own safety.
The injuries that can be caused by travelling with your feet on the dashboard can be devastating and life-changing. It is just not worth the risk.