Any time you have an event that is heavy teen centric, such as a Halloween event, you are increasing your potential for this sort of thing to happen. Some advocate raising prices, which I don't agree with because it would immediately exclude people of a lower income, gang member or not. So if you want to start excluding people, how about raising the age limit to 21 for the event. i'm sure none of you would advocate that.
Businesses are fully aware of the risk when dealing with teens and those that choose to cater to this crowd are willing to take the risk.
BTW, gang members and many lower income folk don't buy season passes. How can you justify buying a pass for the park for next year, when the light bill has to paid tomorrow? Poor folk typically pay full price, (trust me, it's expensive being poor), use discount coupons, or comp tickets. Season passes are a middle class suburban construct.
Some of the ways parks discourage teens from attending, particularly urban teens, is to not play hip hop music in the park, instead they play country and pop. They also name areas; County Fair and BlueRidge Junction, and theme a ride to Nascar. If you want to discourage certain teens, that is the way to do it.