Fair enough, if that is the understood practice.
In the UK we ask in the summary sheet for a specific statement about "The section of the road available to the runners on the day of the race. Pavements?"
n.b. Pavements in this context are what you call sidewalks.
Perhaps US runners are better behaved than UK ones where it is common to see shortcuts being taken on a corner if there is an inviting piece of pavement. So if the organiser does not agree to explicitly ban it in the race instructions, and preferably also tape it off, then I argue to the race organiser that I should measure on the shortest possible line.
Nowadays I tend to use the aerial pictures and birdseye views when certifying a course to check to see if there are potential issues like this which have not been addressed in the measurement report. I guess I was just intrigued by looking at this map of the month and then wondered what it was like on the ground.