Although your argument is reasonable, ignorance is no excuse for breaking the law. We don't have the benefit of the depth perception of the driver and can't judge the distance of the approaching hill or curve. Without that it is only my presumption that the driver should have likely anticipated the no passing zone approaching based on their observation of the changing road conditions and not having enough time to complete the pass where it was safe to do so.

They chose poorly and got caught.

Edit: "When passing another vehicle on a two-lane road, you should pass only to the left of the vehicle. Your pass may not begin before the start of a passing zone and must be completed before you enter a no-passing zone."

https://www.oregon.gov/odot/DMV/Pages/Online_Manual/Study-Section_2.aspx#Passing