We were told by a NPS Ranger this fall that even when you're along the Kolob Terrace Road part of Zion NP (which doesn't have an entrance/fee booth) you are still supposed to purchase a $20 Zion NP 7-day pass from the main entrance. He patrolled and did ask to see people's passes on occasion.
Or, do as I do, buy a $50 NPS pass that's good for an entire year to all National Parks, and then you're covered!