I agree there's a lot of stupidity to the rules, and most rules are knee jerk reactions required because otherwise the next time it happens lawyers will swarm the FAA.
That said, I think it's a bit unreasonable to expect that the Cessna could've turned off the runway at A4. That's about 1500' of runway, at night. If he made it great, but the controller should've confirmed that he made the taxiway. From listening to the tape and running the timeframe in my mind, I would say the cessna was probably still in the flare, or maybe just decelerating when the controller asked him to make A4. He shouldn't have accepted the clearance, and he definitely should've said something to the RJ when he heard the takeoff clearance, but I this (from a regulatory) perspective, the controller is at fault.
IMHO, Regardless of whose at fault, I don't want to die in a fireball caused by someone not checking to see if the runway is clear first. I always listen to other traffic and have even avoided collision because of it. There was a time I was cleared to land and as I turned short final a corporate jet was cleared to TIPH. The jet pilot declined the clearance and I thanked him for listening (I was in a position he couldn't see at the time).
I'll take a few extra rules that I have to follow because the alternative is simply unacceptable. Remember, if the pilot screws up, the pilot dies, if the controller screws up, the pilot still dies. (Unless you're in Switzerland, eh).