Anyone that thinks this is stupid or ineffective is simply dumb.
You cannot reasonably argue the addition of a metal dector won't at least act as a deterrent, and reduce the chances of something catastrophic, intentional or accidental, happening inside the park.
If someone really wants to do some damage, they still can. Without a doubt. It's impossible to be 100% safe. But the increase in security AT A MINIMUM prevents a violent act of happening inside the park gates, where more innocent people are gathered and damage could be done.
In addition, how would you like to crowd into a boat on pirates and a "normal" person sitting next to you who has a concealed carry permit has snuck a gun in his jacket? Welp, they accidentally forgot to put the safety on. Oops. You crowded in, bumped into them and just got shot in the leg.