I heard they did multiple bomb sweeps before the race, so wtf?
You can't stop people from walking on the sidewalk and putting stuff in trashcans.
Also, it's not illegal to carry two backpacks and at a finish line of such a race, there has to be a bunch of supporters there with stuff to give the runners like food, water, clothes, etc. There was probably bags all over the place.
I just don't see how this could be prevented without getting super weird and violating a bunch of rights.