I don’t want this to sound like blaming the victim but anyone in the park at that time was technically not a law abiding citizen. The park was closed. I don’t think it was widely patrolled or enforced, no idea if that changed.
This actually reinforces my point. If you are in an area where people are openly having sexual encounters, doing drugs, etc then -- even if you yourself are not partaking in those activities but are merely breaking a rule about being there -- you are probably not going to look too carefully at anything going on around you.
I live in an urban area and have seen my share of "inappropriate" things happening in streets, parks, subways -- I remain aware of my surroundings as it relates to me but I'm not staring at or memorizing the appearance of these individuals. I am "minding my own business" about these minor illegalities so that I don't draw any criminal attention to myself. Plus I don't want to see public sex acts or people shooting up.
My point was, this helped out the murderer of Katie and Bowie (whether intended or not) because, due to the high likelihood of seeing some private acts in that park, people passing through were not scrutinizing anything too closely.