I haven't actually checked but the story that was told was a guy backed over his 3 years old daughter in Cleveland while using a Yardmachine mower. There is a federal ANSI standard that requires the not mow in reverse unless you have a operator override system.
The, lack of a better term, first generation no mow in reverse wouldn't allow the deck to be running period if the mower was in reverse. The next generation which is what we have now, requires that the operator override the reverse system, providing the mower cannot exceed 1 mile per hour in reverse with the blades running..