I don't disagree with you; the researchers could have taken better safety measures (most notably, better communication between themselves and the reporter would have eliminated most of the risk by allowing the reporter to cut the experiment early), and had they done so, they would have been more clearly in the right.

However, there's some usefulness to the higher speed, since it indicates that the car can be isolated among highway traffic even at high speed. The researchers were also smart to not slam brakes (which would have turned the minimal danger from unpowered coasting into the maximal danger of sudden stops).