I agree, the animal was a threat to others at the zoo as well. When you design for safety, you have to take into account what I call "the moron factor", especially when the moron will put others at risk.
I agree the enclosure should have been higher, but if I were on the jury, I would award the victim family no damages.
The real cause was the guys taunting and mocking the tiger, so despite the deficiency, the zoo shouldn’t have to pay squat. They should be ordered to fix the enclosure, however.