Uhh what? It seems you cannot go a week without reading about a pile-up on a freeway. Just last week a big-rig lost a wheel, it rolled into the on-coming lane, and drivers swerving and braking to avoid it actually caused a pile up. Stopping even on the shoulder on a freeway is considered "risky" by most police officers and many (like triple digits) have been killed while stopped in the shoulder due to vehicles drifting, failing to pay attention, or otherwise being distracted.
I cannot remotely begin to fathom how anyone can think a car going 0-10 MpH on a freeway ISN'T dangerous. And it is absolutely life threatening. If a car behind didn't notice the change in speed, panicked and either hit you or the concrete barrier(s) that could very easily cost them their life. Or leave them with life-long disabilities. Bigger things like trucks and those "road-trains" are even bigger liabilities.
Honestly I'll defend security research strongly in almost all contexts, but when you put people's actual lives in danger you clearly cross a line. There's no shades of gray there, endangering people's lives and health to effectively show off is absolutely immoral and should be illegal (and likely is).
Saying "well nobody got hurt" completely misses the point. It is the intent that is wrong, not the result. The result could have multiplied the wrongness of the intent and resulting in tens of years of jail time, but luckily for them their only "crime" this time was the intent of their dangerous actions.
And let's be frank here luck is the only reason nobody got hurt. The only reason why these two won't be in jail for many years.