The Capital Crescent Trail in the DC area is closed at dusk to all but commuters. there are even a few signs that include the exception for commuters. The signs at major trail entrances usually include a statement of the rules for trail use. I am sure the dusk closure is used the same way here -- to arrest any troublemakers who might hang out on the trail for less than altruistic purposes. You can check out the Capital Crescent Trail website at www.cctrail.org for details.

I can't remember where I saw it (maybe on the website for the American Discovery Trail or on the Annapolis -- Baltimore trail) but someone had some stats to show that bike trails reduce crime by putting more responsible, law abiding folks out there to make criminal activity more difficult. That might be a secondary argument to make to support keeping the trail open at night to commuters.

Good luck.