To me this thread comes down to a simple question: As an adult leader which is more import, personal choice or roll modeling common sense outdoor behavior that the scouts can emulate and succeed at.
While I may wear sandals on a personal trip with my buddies, I'm not going show them off at scout camp if the kids are expected to be in closed shoes for obvious safety reasons. I'll find something that works for me and the guidelines.
I also have no problem telling a parent their kid's chance of completing an outing, healthfully, is going to be less with improper footwear. I believe