I have waited to respond to this since I actually knew one of the "Dancing Grannys" victims. It was Tammy's first parade with the group. Tammy was the daughter of a very good friend at my church during my formative drum corps years. She was also a member of my church youth group my senior year of high school.
Just a few weeks prior to this incident, I was in Chicago and saw the Halloween parade. They had every cross street blocked with trucks.
Having marched hundreds of parades over the years, I would like to think that these events would be better protected. Unfortunately, if someone is hell bent on doing damage, they will find a tool, and a way to make it happen.