the blame will not reinforce the need for safety measures in the future
Well, I think it will in a way. Finding the zoo partly to blame will mean they are required to improve their fencing. Finding the parents partly to blame will (hopefully) encourage other parents to be a bit more careful with their kids.
If it was found to purely the fault of one party, then the other party would feel absolved of any requirement to change.
So whilst assigning blame is in itself unproductive, the indirect effects of that blame being assigned will hopefully go some way to preventing a similar incident occurring in future.