Which is why Dictators are always surrounded by heavily-armed thugs.
But you live in America. Why does a Presidential motorcade have to be scary?
and do not let this post fool you. while i am sure you were able to see the stocks of the rifles (M-4's or MP-5's) up against their shoulders, i can almost guarantee the guns were not raised with the agents looking through the sights at protestors.
i have seen GWB's and Obama's motorcades and worked with secret service during their visits to my city. it's SOP.
it's both a show of force used as a deterrent and being at the ready in case an actual threat does arise.