I have to agree - its for the illusion of safety and the potential for lawsuits. I worked security for 5 years with metal detectors. I can tell you, if you are not doing a full pat down then its doing very little. I assure you there are guns, knives and a host of other contraband things entering the park everyday. Overall, its probably not too dangerous, it makes people feel safe and in court you can always say we have metal detectors. I have never ever felt unsafe in the park. For me the detectors are a blip in my memory of the entire day. I am at the security area for less than 2 min.
my 2 cents
Kevin