Fragola - I think there is a difference in the risk between a 5 yo cycling to school unsupervised and the dangers of overeating or not exercising enough.
There is a risk of injury, here and now with the cycling which is not present with the overeating or lack of exercise - if there is a risk of injury from those things then they are years down the line and could be increased or reduced by all manner of other variables as the child grows up.
It maybe there have been a few other children who have been hurt on the way in to school and it is a dangerous area and the school don't know what else to do to get through to the parents that it isn't wise to leave their children like this. It depends, for me, whether the parents are making a stand for the sake of making a stand on this rather than doing what is best for their children.
Most people wouldn't leave their 5 yr old alone in the house and yet it is deemed OK by these parents to let a five yr old ride a bike on busy roads which seems to me to be infinitely more dangerous. Just the thought of it makes me shudder. No car would see a 5 yr old if they were close and even adults on the pavement may step in front of a child on a bike. The risks just seem too high.