what's to stop me and 4 friends from jumping out in front of the cars just to laugh as it crashes itself to "save" us.
This example is one of "malicious behavior", which is an issue for the courts. With any luck the "Just for laughs" comment would reach the judge.
It is not an example of malicious input. The car correctly sensed a risk to human life/health and correctly identified the best alternative to maintain its"prime directive". The vehicle's decision would have been exactly correct (presuming there were no better alternatives, such as stopping). An example of "malformed/malicious input" would be when the side of a truck gets confused for an overhead sign,