Because in the rules: an aid station worker and finish line chute official are deemed "event personnel" and are not considered "outside assistance". The Aid station workers at the ITU Grand Final aren't the same dummies who man the water stations at the Grandma's Pancake House 5k. They are event staff. They cannot help an athlete progress through the course.
When Ali saw this guy grabbing Jonny, he instantly pushed him off and took him up himself. That is because Ali knew that if that official took one step forward with Jonny on his shoulder, Jonny would have gotten a DNF. But another athlete can help another competitor across the line as long as they are able to finish the race as well.