There's a huge difference between stopping on the highway and decelerating due to lack of engine power. The driver knew what was happening, turned on his hazard lights, and didn't apply the brakes. Slowing down on the highway, although annoying, shouldn't be an unfamiliar or unsafe scenario (ex: construction, traffic backup, etc.)

This would be a completely different story if the researchers applied full force to the brakes or accelerator since those are unexpected (to other drivers), sudden, and difficult to react to behaviours.

Everything else aside, slowing without good reason is likely to be a traffic infraction (in Missouri, a misdemeanor punishable by 1 year in jail!).

It isn't that convoluted to hold the driver responsible for the vehicle, they knew prior to driving into the area with a minimum speed that there was some intent to tamper with it.