I can't think of any coaster where you *must* hold on. Well, except for Mean Streak, but that has more to do with self-preservation than with ride dynamics.
The problem is that lawyers cannot properly discern the difference between "hands up" and "hands OUT". "Keep hands inside the car at all times" means exactly that, and from a legal standpoint, that invisible bubble over the train that most people use to define "inside" simply does not exist. If the hands are above the seating compartment then clearly they are not inside the car. "If they are not inside the car in the advertisement," the argument goes, "then how can you blame my client for sticking his hand out and getting it mangled by the ride structure?" Unfortunately the "up out" argument doesn't hold up in court.
It's a real shame, too.
--Dave Althoff, Jr.