It is a stupid cheap halfway enforcement because of parks at night being a good place for vice activities which leave unsavory evidence later (used syringes, condoms, booze bottles).
I know because that was a shift taken after a few incidents of prostitution at night and unsavory hazardous litter prompted the change.
The alternatives are obvious (open but larger patrols at night to make it unsuitable) but that is more expensive for what is a fringe activity.
