I'd have exactly the same attitude. I wouldn't hold the park at fault regardless of who got hurt, even if it was me let alone family or friends, not unless they gave them the knife or set up the fight. This is one of the major things wrong with the world in my eyes; everything is always someone else fault. Someone should have prevented, someone should have intervened, someone should have seen the future. The suspect is the suspect.
You can mitigate against a risk, but you can not eliminate all danger. The security as it is is a reasonable response to the risk presented. With this being the first incident of this type in 40 years, how many issues have they prevented with their actions in this time? Loads I expect. Considering your opinion of their clientele you'd be hard pressed to argue against that having been the case, but you can't prove the effectiveness of a preventative measure.
You can learn from incidents, you can see how something happened and amend your processes for the future, but that does not prove an ability to have foreseen.